FM31L278/FM31L276
3V Integrated Processor Companion with Memory
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 1 of 26
Features
High Integration Device Replaces Mul ti pl e Parts
• Serial Nonvolatile Memory
• Real-time Clock (RTC)
• Low Voltage Reset
• Watchdog Timer
• Early Power-Fail Warning/NMI
• Two 16-bit Event Counters
• Serial Number with Write-lock for Security
Ferroelectric Nonvolatile RAM
• 64Kb, and 256Kb versions
• Virtually Unlimited Read/Write Endurance
• 38 year Data Retention (+75°C)
• NoDelay™ Writes
Real-time Clock/Calendar
• Backup Current at 2V, 1.15 μA (max.) at +25C
• Seconds through Centuries in BCD format
• Tracks Leap Years through 2099
• Uses Standard 32.768 kHz Crystal (12.5pF)
• Software Calibration
• Supports Battery or Capacitor Backup
Processor Companion
• Active-low Reset Output for VDD and Watchdog
• Programmable VDD Reset Trip Point
• Manual Reset Filtered and Debounced
• Programmable Watchdog Timer
• Dual Battery-backed Event Counter Tracks
System Intrusions or other Events
• Comparator for Early Power-Fail Interrupt
• 64-bit Programmable Serial Number with Lock
Fast Two-wire Serial Interface
• Up to 1 MHz Maximum Bus Frequency
• Supports Legacy Timing for 100 kHz & 400 kHz
• Device Select Pins for up to 4 Memory Devices
• RTC, Supervisor Controlled via 2-wire Interface
Easy to Use Configurations
• Operates from 2.7 to 3.6V
• 14-pin “Green”/RoHS SOIC package (-G)
• Low Operating Current
• -40°C to +85°C Operation
• Underwriters Laboratory (UL) Recognized
Description
The FM31L27x is a family of integrated devices that
includes the most commonly needed functions for
processor-based systems. Major features include
nonvolatile memory available in various sizes, real-
time clock, low-VDD reset, watchdog timer,
nonvolatile event counter, lockable 64-bit serial
number area, and general purpose comparator that
can be used for an early power-fail (NMI) interrupt or
other purpose. The family operates from 2.7 to 3.6V.
Each FM31L27x provides nonvolatile RAM
available in sizes including 64Kb, and 256Kb
versions. Fast write speed and unlimited endurance
allow the memory to serve as extra RAM or
conventional nonvolatile storage. This memory is
truly nonvolatile rather than battery backed.
The real-time clock (RTC) provides time and date
information in BCD format. It can be permanently
powered from external backup voltage source, either
a battery or a capacitor. The timekeeper uses a
common external 32.768 kHz crystal and provides a
calibration mode that allows software adjustment of
timekeeping accuracy.
The processor companion includes commonly needed
CPU support functions. Supervisory functions
include a reset output signal controlled by either a
low VDD condition or a watchdog timeout. /RST
goes active when VDD drops below a programmable
threshold and remains active for 100 ms after VDD
rises above the trip point. A programmable watchdog
timer runs from 100 ms to 3 seconds. The watchdog
timer is optional, but if enabled it will assert the reset
signal for 100 ms if not restarted by the host before
the timeout. A flag-bit indicates the source of the
reset.
A general-purpose comparator compares an external
input pin to the onboard 1.2V reference. This is
useful for generating a power-fail interrupt (NMI) but
can be used for any purpose. The family also includes
a programmable 64-bit serial number that can be
locked making it unalterable. Additionally it offers a
dual battery-backed event counter that tracks the
number of rising or falling edges detected on
dedicated input pins.
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 2 of 26
Pin Configuration
Pin Name Function
CNT1, CNT2 Event Counter Inputs
A0, A1 Device Select inputs
CAL/PFO Clock Calibration and Early
Power-Fail Output
/RST Reset Input/Output
PFI Early Power-fail Input
X1, X2 Crystal Connections
SDA Serial Data
SCL Serial Clock
VBAK Battery-Backup Supply
VDD Supply Voltage
VSS Ground
Ordering Information
Base Configuration Memory Size Operating Voltage Reset Threshold Ordering Part Number
FM31L278 256Kb 2.7-3.6V 2.6V, 2.9V FM31L278-G
256Kb 2.7-3.6V 2.6V, 2.9V FM31L278-GTR (tape&reel)
FM31L276 64Kb 2.7-3.6V 2.6V, 2.9V FM31L276-G
64Kb 2.7-3.6V 2.6V, 2.9V FM31L276-GTR (tape&reel)
VDD
VBAK
SCL
SDA
VSS
X1
X2
CAL/PFO
CNT1
PFI
RST
A0
A1
CNT2
1
2
3
4
5
6
7
14
13
12
11
10
9
8
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 3 of 26
Figure 1. Block Diagram
Pin Descriptions
Pin Name Type Pin Description
A0, A1 Input Device select inputs are used to address multiple memories on a serial bus. To select the
device the address value on the two pins must match the corresponding bits contained in
the device address. The device select pins are pulled down internally.
CNT1,
CNT2
Input Event Counter Inputs: These battery-backed inputs increment counters when an edge is
detected on the corresponding CNT pin. The polarity is programmable. These pins
should not be left floating. Tie to ground if these pins are not used.
CAL/PFO Output In calibration mode, this pin supplies a 512 Hz square-wave output for clock calibration.
In normal operation, this is the early power-fail output.
X1, X2 I/O 32.768 kHz crystal connection. When using an external oscillator, apply the clock to X1
and a DC mid-level to X2 (see Crystal Oscillator section for suggestions).
/RST I/O Active low reset output with weak pull-up. Also input for manual reset.
SDA I/O Serial Data & Address: This is a bi-directional line for the two-wire interface. It is open-
drain and is intended to be wire-OR’d with other devices on the two-wire bus. The input
buffer incorporates a Schmitt trigger for noise immunity and the output driver includes
slope control for falling edges. A pull-up resistor is required.
SCL Input Serial Clock: The serial clock line for the two-wire interface. Data is clocked out of the
part on the falling edge, and in on the rising edge. The SCL input also incorporates a
Schmitt trigger input for noise immunity.
PFI Input Early Power-fail Input: Typically connected to an unregulated power supply to detect an
early power failure. This pin should not be left floating.
VBAK Supply Backup supply voltage: A 3V battery or a large value capacitor. If no backup supply is
used, this pin should be tied to ground and the VBC bit should be cleared. The trickle
charger is UL recognized and ensures no excessive current when using a lithium battery.
VDD Supply Supply Voltage
VSS Supply Ground
F-RAM
Array
2-Wire
Interface
SCL
SDA
RST
A1, A0
CAL/PFO
PFI
VDD
VBAK
RTC
2.5V -
+
RTC Re
g
isters
Event
Counters
CNT1
CNT2
Special
Function
Registers
S/N X1
X2
LockOut
LockOut
+
- 1.2V
Watchdog
LV Detect
Switched Powe
r
512Hz
RTC Cal.
Battery Backed
Nonvolatile
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 4 of 26
Overview
The FM31L27x family combines a serial nonvolatile
RAM with a real-time clock (RTC) and a processor
companion. The companion is a highly integrated
peripheral including a processor supervisor, a
comparator used for early power-fail warning,
nonvolatile event counters, and a 64-bit serial
number. The FM31L27x integrates these
complementary but distinct functions that share a
common interface in a single package. Although
monolithic, the product is organized as two logical
devices, the F-RAM memory and the
RTC/companion. From the system perspective they
appear to be two separate devices with unique IDs on
the serial bus.
The memory is organized as a stand-alone 2-wire
nonvolatile memory with a standard device ID value.
The real-time clock and supervisor functions are
accessed with a separate 2-wire device ID. This
allows clock/calendar data to be read while
maintaining the most recently used memory address.
The clock and supervisor functions are controlled by
25 special function registers. The RTC and event
counter circuits are maintained by the power source
on the VBAK pin, allowing them to operate from
battery or backup capacitor power when VDD drops
below an internally set threshold. Each functional
block is described below.
Memory Operation
The FM31L27x is a family of products available in
different memory sizes including 64Kb, and 256Kb.
The family is software compatible, all versions use
consistent two-byte addressing for the memory
device. This makes the lowest density device
different from its stand-alone memory counterparts
but makes them compatible within the entire family.
Memory is organized in bytes, for example the 64Kb
memory is 8192 x 8 and the 256Kb memory is 32768
x 8. The memory is based on F-RAM technology.
Therefore it can be treated as RAM and is read or
written at the speed of the two-wire bus with no
delays for write operations. It also offers effectively
unlimited write endurance unlike other nonvolatile
memory technologies. The 2-wire interface protocol
is described further on page 13.
The memory array can be write-protected by
software. Two bits in the processor companion area
(WP0, WP1 in register 0Bh) control the protection
setting as shown in the following table. Based on the
setting, the protected addresses cannot be written and
the 2-wire interface will not acknowledge any data to
protected addresses. The special function registers
containing these bits are described in detail below.
Write protect addresses WP1 WP0
None 0 0
Bottom 1/4 0 1
Bottom 1/2 1 0
Full array 1 1
Processor Companion
In addition to nonvolatile RAM, the FM31L27x
family incorporates a highly integrated processor
companion. It includes a low voltage reset, a
programmable watchdog timer, battery-backed event
counters, a comparator for early power-fail detection
or other purposes, and a 64-bit serial number.
Processor Supervisor
Supervisors provide a host processor two basic
functions: detection of power supply fault conditions
and a watchdog timer to escape a software lockup
condition. All FM31L27x devices have a reset pin
(/RST) to drive the processor reset input during
power faults (and power-up) and software lockups. It
is an open drain output with a weak internal pull-up
to VDD. This allows other reset sources to be wire-
OR’d to the /RST pin. When VDD is above the
programmed trip point, /RST output is pulled weakly
to VDD. If VDD drops below the reset trip point
voltage level (VTP) the /RST pin will be driven low. It
will remain low until VDD falls too low for circuit
operation which is the VRST level. When VDD rises
again above VTP, /RST will continue to drive low for
at least 100 ms (tRPU) to ensure a robust system reset
at a reliable VDD level. After tRPU has been met, the
/RST pin will return to the weak high state. While
/RST is asserted, serial bus activity is locked out even
if a transaction occurred as VDD dropped below VTP.
A memory operation started while VDD is above VTP
will be completed internally.
Figure 2 below illustrates the reset operation in
response to the VDD voltage.
Figure 2. Low Voltage Reset
VDD
VTP tRPU
RST
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 5 of 26
The bit VTP controls the trip point of the low voltage
detect circuit. It is located in register 0Bh, bit 0. Note
that the bit 1 location is a “don’t care”.
VTP VTP
2.6V 0
2.9V 1
The watchdog timer can also be used to assert the
reset signal (/RST). The watchdog is a free running
programmable timer. The period can be software
programmed from 100 ms to 3 seconds in 100 ms
increments via a 5-bit nonvolatile register. All
programmed settings are minimum values and vary
with temperature according to the operating
specifications. The watchdog has two additional
controls associated with its operation, a watchdog
enable bit (WDE) and timer restart bits (WR). Both
the enable bit must be set and the watchdog must
timeout in order to drive /RST active. If a reset event
occurs, the timer will automatically restart on the
rising edge of the reset pulse. If WDE=0, the
watchdog timer runs but a watchdog fault will not
cause /RST to be asserted low. The WTR flag will be
set, indicating a watchdog fault. This setting is useful
during software development and the developer does
not want /RST to drive. Note that setting the
maximum timeout setting (11111b) disables the
counter to save power. The second control is a nibble
that restarts the timer preventing a reset. The timer
should be restarted after changing the timeout value.
The watchdog timeout value is located in register
0Ah, bits 4-0, and the watchdog enable is bit 7. The
watchdog is restarted by writing the pattern 1010b to
the lower nibble of register 09h. Writing this pattern
will also cause the timer to load new timeout values.
Writing other patterns to this address will not affect
its operation. Note the watchdog timer is free-
running. Prior to enabling it, users should restart the
timer as described above. This assures that the full
timeout period will be set immediately after enabling.
The watchdog is disabled when VDD is below VTP.
The following table summarizes the watchdog bits. A
block diagram follows.
Watchdog timeout WDT4-0 0Ah, bits 4-0
Watchdog enable WDE 0Ah, bit 7
Watchdog restart WR3-0 09h, bits 3-0
Figure 3. Watchdog Timer
Manual Reset
The /RST pin is bi-directional and allows the
FM31L27x to filter and de-bounce a manual reset
switch. The /RST input detects an external low
condition and responds by driving the /RST signal
low for 100 ms.
Figure 4. Manual Reset
Note that an internal weak pull-up on /RST
eliminates the need for additional external
components.
Reset Flags
In case of a reset condition, a flag will be set to
indicate the source of the reset. A low VDD reset is
indicated by the POR flag, register 09h bit 6. A
watchdog reset is indicated by the WTR flag, register
09h bit 7. Note that the flags are internally set in
response to reset sources, but they must be cleared by
the user. When the register is read, it is possible that
both flags are set if both have occurred since the user
last cleared them.
Early Power Fail Comparator
An early power fail warning can be provided to the
processor well before VDD drops out of spec. The
comparator is used to create a power fail interrupt
(NMI). This can be accomplished by connecting the
PFI pin to the unregulated power supply via a resistor
divider. An application circuit is shown below.
FM31L27x
Reset
Switch
RST
MCU
Switch
Behavio
r
RST FM31L27x
drives
100 ms (min.)
Timebase Counter
Watchdog
timeout
100 ms
cloc
k
WDE
/
RST
WR3-0 = 1010b to restart
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 6 of 26
Figure 5. Comparator as Early Power-Fail Warning
The voltage on the PFI input pin is compared to an
onboard 1.2V reference. When the PFI input voltage
drops below this threshold, the comparator will drive
the CAL/PFO pin to a low state. The comparator has
100 mV (max) of hysteresis to reduce noise
sensitivity, only for a rising PFI signal. For a falling
PFI edge, there is no hysteresis.
The comparator is a general purpose device and its
application is not limited to the NMI function.
The comparator is not integrated into the special
function registers except as it shares its output pin
with the CAL output. When the RTC calibration
mode is invoked by setting the CAL bit (register 00h,
bit 2), the CAL/PFO output pin will be driven with a
512 Hz square wave and the comparator will be
ignored. Since most users only invoke the calibration
mode during production, this should have no impact
on system operations using the comparator.
Note: The maximum voltage on the comparator input PFI
is limited to 3.75V under normal operating conditions.
Event Counter
The FM31L27x offers the user two battery-backed
event counters. Input pins CNT1 and CNT2 are
programmable edge detectors. Each clocks a 16-bit
counter. When an edge occurs, the counters will
increment their respective registers. Counter 1 is
located in registers 0Dh and 0Eh, Counter 2 is
located in registers 0Fh and 10h. These register
values can be read anytime VDD is above VTP, and
they will be incremented as long as a valid VBAK
power source is provided. To read, set the RC bit
register 0Ch bit 3 to 1. This takes a snapshot of all
four counter bytes allowing a stable value even if a
count occurs during the read. The registers can be
written by software allowing the counters to be
cleared or initialized by the system. Counts are
blocked during a write operation. The two counters
can be cascaded to create a single 32-bit counter by
setting the CC control bit (register 0Ch). When
cascaded, the CNT1 input will cause the counter to
increment. CNT2 is not used in this mode.
Figure 6. Event Counter
The control bits for event counting are located in
register 0Ch. Counter 1 Polarity is bit C1P, bit 0;
Counter 2 Polarity is C2P, bit 1; the Cascade Control
is CC, bit 2; and the Read Counter bit is RC bit 3.
The polarity bits must be set prior to setting the
counter value(s). If a polarity bit is changed, the
counter may inadvertently increment. If the counter
pins are not being used, tie them to ground.
Serial Number
A memory location to write a 64-bit serial number is
provided. It is a writeable nonvolatile memory block
that can be locked by the user once the serial number
is set. The 8 bytes of data and the lock bit are all
accessed via the device ID for the processor
companion. Therefore the serial number area is
separate and distinct from the memory array. The
serial number registers can be written an unlimited
number of times, so these locations are general
purpose memory. However once the lock bit is set the
values cannot be altered and the lock cannot be
removed. Once locked the serial number registers can
still be read by the system.
The serial number is located in registers 11h to 18h.
The lock bit is SNL, register 0Bh bit 7. Setting the
SNL bit to a 1 disables writes to the serial number
registers, and the SNL bit cannot be cle are d.
Real-Time Clock Operation
The real-time clock (RTC) is a timekeeping device
that can be battery or capacitor backed for
permanently-powered operation. It offers a software
calibration feature that allows high accuracy.
The RTC consists of an oscillator, clock divider, and
a register system for user access. It divides down the
32.768 kHz time-base and provides a minimum
16-bit Counte
r
CNT1
CC
CNT2
C1P
C2P
16-bit Counte
r
+
-1.2V ref
Regulato
r
VDD
FM31L27x
To MCU
NMI input CAL/PFO PFI
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 7 of 26
resolution of seconds (1Hz). Static registers provide
the user with read/write access to the time values. It
includes registers for seconds, minutes, hours, day-
of-the-week, date, months, and years. A block
diagram (Figure 7) illustrates the RTC function.
The user registers are synchronized with the
timekeeper core using R and W bits in register 00h
described below. Changing the R bit from 0 to 1
transfers timekeeping information from the core into
holding registers that can be read by the user. If a
timekeeper update is pending when R is set, then the
core will be updated prior to loading the user
registers. The registers are frozen and will not be
updated again until the R bit is cleared to 0. R is used
for reading the time.
Setting the W bit to 1 locks the user registers.
Clearing it to 0 causes the values in the user registers
to be loaded into the timekeeper core. W is used for
writing new time values. Users should be certain not
to load invalid values, such as FFh, to the
timekeeping registers. Updates to the timekeeping
core occur continuously except when locked.
Backup Power
The real-time clock/calendar is intended to be
permanently powered. When the primary system
power fails, the voltage on the VDD pin will drop.
When VDD is less 2.5V the RTC (and event counters)
will switch to the backup power supply on VBAK. The
clock operates at extremely low current in order to
maximize battery or capacitor life. However, one of
the advantages of combining a clock function with
the F-RAM memory is that data is not lost regardless
of the backup power source.
The IBAK current varies with temperature and voltage
(see DC parametric table). The following graph
shows IBAK as a function of VBAK. These curves are
useful for calculating backup time when a capacitor
is used as the VBAK source.
Figure 7. IBAK vs. VBAK Volt age
The minimum VBAK voltage varies linearly with
temperature. The user can expect the minimum VBAK
voltage to be 1.23V at +85°C and 1.90V at -40°C.
The tested limit is 1.55V at +25°C. The minimum
VBAK voltage has been characterized at -40°C and
+85°C but is not 100% tested.
Figure 8. VBAK (min.) vs. Temperature
Trickle Charger
To facilitate capacitor backup the VBAK pin can
optionally provide a trickle charge current. When the
VBC bit, register 0Bh bit 2, is set to ‘1’, the VBAK pin
will source approximately 80 µA until VBAK reaches
VDD. This charges the capacitor to VDD without an
external diode and resistor charger. There is a Fast
Charge mode which is enabled by the FC bit (register
0Bh, bit 5). In this mode the trickle charger current is
set to approximately 1 mA, allowing a large backup
capacitor to charge more quickly.
In the case where no backup supply is used, the VBAK
pin should be tied to VSS. Be sure to turn off the
trickle charger (VBC=0), otherwise charger current
will be shunted to ground from VDD.
, Note: systems using lithium batteries should clear
the VBC bit to 0 to prevent battery charging. The
VBAK circuitry includes an internal 1 K
Ω
series
resistor as a safety element. The trickle charger is UL
Recognized.
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 8 of 26
32.768 kH
z
Oscillato
r
Cloc
k
Divide
r
Update
Logi
c
512 Hz W
R
Seconds
7 bits
Minutes
7 bits
Hours
6 bits
Date
6 bits
Months
5 bits
Years
8 bits
CF
Days
3 bits
User Interface Registers
1 Hz
/OSCEN
Figure 9. Real-Time Clock Core Block Diagram
Calibration
When the CAL bit in a register 00h is set to 1, the
clock enters calibration mode. In calibration mode,
the CAL/PFO output pin is dedicated to the
calibration function and the power fail output is
temporarily unavailable. Calibration operates by
applying a digital correction to the counter based on
the frequency error. In this mode, the CAL/PFO pin
is driven with a 512 Hz (nominal) square wave. Any
measured deviation from 512 Hz translates into a
timekeeping error. The user converts the measured
error in ppm and writes the appropriate correction
value to the calibration register. The correction
factors are listed in the table below. Positive ppm
errors require a negative adjustment that removes
pulses. Negative ppm errors require a positive
correction that adds pulses. Positive ppm adjustments
have the CALS (sign) bit set to 1, where as negative
ppm adjustments have CALS = 0. After calibration,
the clock will have a maximum error of ± 2.17 ppm
or ± 0.09 minutes per month at the calibrated
temperature.
The calibration setting is stored in F-RAM so is not
lost should the backup source fail. It is accessed with
bits CAL.4-0 in register 01h. This value only can be
written when the CAL bit is set to a 1. To exit the
calibration mode, the user must clear the CAL bit to a
0. When the CAL bit is 0, the CAL/PFO pin will
revert to the power fail output function.
Crystal Oscillator
The crystal oscillator is designed to use a 12.5pF
crystal without the need for external components,
such as loading capacitors. The FM31L27x device
has built-in loading capacitors that match the crystal.
If a 32.768kHz crystal is not used, an external
oscillator may be connected to the FM31L27x.
Apply the oscillator to the X1 pin. Its high and low
voltage levels can be driven rail-to-rail or amplitudes
as low as approximately 500mV p-p. To ensure
proper operation, a DC bias must be applied to the
X2 pin. It should be centered between the high and
low levels on the X1 pin. This can be accomplished
with a voltage divider.
In the example, R1 and R2 are chosen such that the
X2 voltage is centered around the X1 oscillator drive
levels. If you wish to avoid the DC current, you may
choose to drive X1 with an external clock and X2
with an inverted clock using a CMOS inverter.
Figure 10. External Oscillator
X1 X2 Vd
d
FM31L27x
R1
R2
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 9 of 26
Layout Requirements
The X1 and X2 crystal pins employ very high
impedance circuits and the oscillator connected to
these pins can be upset by noise or extra loading. To
reduce RTC clock errors from signal switching noise,
a guard ring must be placed around these pads and
the guard ring grounded. SDA and SCL traces should
be routed away from the X1/X2 pads. The X1 and X2
trace lengths should be less than 5 mm. The use of a
ground plane on the backside or inner board layer is
preferred. See layout example. Red is the top layer,
green is the bottom layer.
Layout for Surface Mount Crystal Layout for Through Hole Crystal
(red = top layer, green = bottom layer) (red = top layer, green = bottom layer
Calibration Adjus tmen ts
Positive Calibration for slow clocks: Calibration will achieve
±
2.17 PPM afte r calibration
Measured Frequency Range Error Range (PPM)
Min Max Min Max Program Calibration Register to:
0 512.0000 511.9989 0 2.17 000000
1 511.9989 511.9967 2.18 6.51 100001
2 511.9967 511.9944 6.52 10.85 100010
3 511.9944 511.9922 10.86 15.19 100011
4 511.9922 511.9900 15.20 19.53 100100
5 511.9900 511.9878 19.54 23.87 100101
6 511.9878 511.9856 23.88 28.21 100110
7 511.9856 511.9833 28.22 32.55 100111
8 511.9833 511.9811 32.56 36.89 101000
9 511.9811 511.9789 36.90 41.23 101001
10 511.9789 511.9767 41.24 45.57 101010
11 511.9767 511.9744 45.58 49.91 101011
12 511.9744 511.9722 49.92 54.25 101100
13 511.9722 511.9700 54.26 58.59 101101
14 511.9700 511.9678 58.60 62.93 101110
15 511.9678 511.9656 62.94 67.27 101111
16 511.9656 511.9633 67.28 71.61 110000
17 511.9633 511.9611 71.62 75.95 110001
18 511.9611 511.9589 75.96 80.29 110010
19 511.9589 511.9567 80.30 84.63 110011
20 511.9567 511.9544 84.64 88.97 110100
21 511.9544 511.9522 88.98 93.31 110101
22 511.9522 511.9500 93.32 97.65 110110
23 511.9500 511.9478 97.66 101.99 110111
24 511.9478 511.9456 102.00 106.33 111000
25 511.9456 511.9433 106.34 110.67 111001
26 511.9433 511.9411 110.68 115.01 111010
27 511.9411 511.9389 115.02 119.35 111011
28 511.9389 511.9367 119.36 123.69 111100
29 511.9367 511.9344 123.70 128.03 111101
30 511.9344 511.9322 128.04 132.37 111110
31 511.9322 511.9300 132.38 136.71 111111
VDD
SCL
SDA
X2
X1
PFI
VBAK
VDD
SCL
SDA
X2
X1
PFI
VBAK
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
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Rev. 3.0
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Negative Calibration for fast clocks: Calibrati on will achieve
±
2.17 PPM afte r calibration
Measured Frequency Range Error Range (PPM)
Min Max Min Max Program Calibration Register to:
0 512.0000 512.0011 0 2.17 000000
1 512.0011 512.0033 2.18 6.51 000001
2 512.0033 512.0056 6.52 10.85 000010
3 512.0056 512.0078 10.86 15.19 000011
4 512.0078 512.0100 15.20 19.53 000100
5 512.0100 512.0122 19.54 23.87 000101
6 512.0122 512.0144 23.88 28.21 000110
7 512.0144 512.0167 28.22 32.55 000111
8 512.0167 512.0189 32.56 36.89 001000
9 512.0189 512.0211 36.90 41.23 001001
10 512.0211 512.0233 41.24 45.57 001010
11 512.0233 512.0256 45.58 49.91 001011
12 512.0256 512.0278 49.92 54.25 001100
13 512.0278 512.0300 54.26 58.59 001101
14 512.0300 512.0322 58.60 62.93 001110
15 512.0322 512.0344 62.94 67.27 001111
16 512.0344 512.0367 67.28 71.61 010000
17 512.0367 512.0389 71.62 75.95 010001
18 512.0389 512.0411 75.96 80.29 010010
19 512.0411 512.0433 80.30 84.63 010011
20 512.0433 512.0456 84.64 88.97 010100
21 512.0456 512.0478 88.98 93.31 010101
22 512.0478 512.0500 93.32 97.65 010110
23 512.0500 512.0522 97.66 101.99 010111
24 512.0522 512.0544 102.00 106.33 011000
25 512.0544 512.0567 106.34 110.67 011001
26 512.0567 512.0589 110.68 115.01 011010
27 512.0589 512.0611 115.02 119.35 011011
28 512.0611 512.0633 119.36 123.69 011100
29 512.0633 512.0656 123.70 128.03 011101
30 512.0656 512.0678 128.04 132.37 011110
31 512.0678 512.0700 132.38 136.71 011111
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 11 of 26
Register Map
The RTC and processor companion functions are accessed via 25 special function registers mapped to a separate 2-
wire device ID. The interface protocol is described below. The registers contain timekeeping data, control bits, or
information flags. A description of each register follows the summary table below.
Register Map Summary Table
Nonvolatile = Battery-backed =
Address D7 D6 D5 D4 D3 D2 D1 D0 Function Range
Serial Number Byte 7 Serial Number 7 FFh
Serial Number Byte 6 Serial Number 6 FFh
Serial Number Byte 5 Serial Number 5 FFh
Serial Number Byte 4 Serial Number 4 FFh
Serial Number Byte 3 Serial Number 3 FFh
Serial Number Byte 2 Serial Number 2 FFh
Serial Number Byte 1 Serial Number 1 FFh
Serial Number Byte 0 Serial Number 0 FFh
10h Counter 2 MSB Event Counter 2 MSB FFh
0Fh Counter 2 LSB Event Counter 2 LSB FFh
0Eh Counter 1 MSB Event Counter 1 MSB FFh
0Dh Counter 1 LSB Event Counter 1 LSB FFh
0Ch RC CC C2P C1P Event Count Control
0Bh SNL - FC WP1 WP0 VBC - VT P Companion Control
0Ah WDE - - WDT4 WDT3 WDT2 WDT1 WDT0 Watchdog Control
09h WTR POR LB - WR3 WR2 WR1 WR0 Watchdog Restart/Flags
08h 10 years years Years 00-99
07h 00010 mo months Month 1-12
06h 00 10 date date Date 1-31
05h 00000 day Da
y
1-7
04h 00 10 hours hours Hours 0-23
03h 010 minutes minutes Minutes 0-59
02h 010 seconds seconds Seconds 0-59
01h /OSCEN reserved CALS CAL4 CAL3 CAL2 CAL1 CAL0 CAL/Control
00h reserved CF reserved reserved reserved CAL W R RTC Control
18h
17h
11h
16h
15h
14h
13h
12h
Note: When the device is first powered up and programmed, all registers must be written because the battery-
backed register values cannot be guaranteed. The table below shows the default values of the non-volatile
registers. All other register values should be treated as unknown.
Default Register Values
Address Hex Value
18h 0x00
17h 0x00
16h 0x00
15h 0x00
14h 0x00
13h 0x00
12h 0x00
11h 0x00
0Bh 0x00
0Ah 0x1F
01h 0x80
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
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Register Description
Address Description
18h Serial Number Byte 7
D7 D6 D5 D4 D3 D2 D1 D0
SN.63 SN.62 SN.61 SN.60 SN.59 SN.58 SN.57 SN.56
Upper byte of the serial number. Read/write when SNL=0, read-only when SNL=1. Nonvolatile.
17h Serial Number Byte 6
D7 D6 D5 D4 D3 D2 D1 D0
SN.55 SN.54 SN.53 SN.52 SN.51 SN.50 SN.49 SN.48
Byte 6 of the serial number. Read/write when SNL=0, read-only when SNL=1. Nonvolatile.
16h Serial Number Byte 5
D7 D6 D5 D4 D3 D2 D1 D0
SN.47 SN.46 SN.45 SN.44 SN.43 SN.42 SN.41 SN.40
Byte 5 of the serial number. Read/write when SNL=0, read-only when SNL=1. Nonvolatile.
15h Serial Number Byte 4
D7 D6 D5 D4 D3 D2 D1 D0
SN.39 SN.38 SN.37 SN.36 SN.35 SN.34 SN.33 SN.32
Byte 4 of the serial number. Read/write when SNL=0, read-only when SNL=1. Nonvolatile.
14h Serial Number Byte 3
D7 D6 D5 D4 D3 D2 D1 D0
SN.31 SN.30 SN.29 SN.28 SN.27 SN.26 SN.25 SN.24
Byte 3 of the serial number. Read/write when SNL=0, read-only when SNL=1. Nonvolatile.
13h Serial Number Byte 2
D7 D6 D5 D4 D3 D2 D1 D0
SN.23 SN.22 SN.21 SN.20 SN.19 SN.18 SN.17 SN.16
Byte 2 of the serial number. Read/write when SNL=0, read-only when SNL=1. Nonvolatile.
12h Serial Number Byte 1
D7 D6 D5 D4 D3 D2 D1 D0
SN.15 SN.14 SN.13 SN.12 SN.11 SN.10 SN.9 SN.8
Byte 1 of the serial number. Read/write when SNL=0, read-only when SNL=1. Nonvolatile.
11h Serial Number Byte 0
D7 D6 D5 D4 D3 D2 D1 D0
SN.7 SN.6 SN.5 SN.4 SN.3 SN.2 SN.1 SN.0
LSB of the serial number. Read/write when SNL=0, read-only when SNL=1. Nonvolatile.
10h Counter 2 MSB
D7 D6 D5 D4 D3 D2 D1 D0
C2.15 C2.14 C2.13 C2.12 C2.11 C2.10 C2.9 C2.8
Event Counter 2 MSB. Increments on overflows from Counter 2 LSB. Battery-backed, read/write.
0Fh Counter 2 LSB
D7 D6 D5 D4 D3 D2 D1 D0
C2.7 C2.6 C2.5 C2.4 C2.3 C2.2 C2.1 C2.0
Event Counter 2 LSB. Increments on programmed edge event on CNT2 input or overflows from Counter 1 MSB
when CC=1. Battery-backed, read/write .
0Eh Counter 1 MSB
D7 D6 D5 D4 D3 D2 D1 D0
C1.15 C1.14 C1.13 C1.12 C1.11 C1.10 C1.9 C1.8
Event Counter 1 MSB. Increments on overflows from Counter 1 LSB. Battery-backed, read/write.
0Dh Counter 1 LSB
D7 D6 D5 D4 D3 D2 D1 D0
C1.7 C1.6 C1.5 C1.4 C1.3 C1.2 C1.1 C1.0
Event Counter 1 LSB. Increments on programmed edge event on CNT1 input. Battery-backed, read/write.
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
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Rev. 3.0
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0Ch Event Counter Control
D7 D6 D5 D4 D3 D2 D1 D0
- - - - RC CC C2P C1P
RC Read Counter. Setting this bit to 1 takes a snapshot of the four counters bytes allowing the system to read the
values without missing count events. The RC bit will be automatically cleared.
CC Counter Cascade. When CC=0, the event counters operate independently according to the edge programmed by
C1P and C2P respectively. When CC=1, the counters are cascaded to create one 32-bit counter. The registers of
Counter 2 represent the most significant 16-bits of the counter and CNT1 is the controlling input. Bit C2P is
“don’t care” when CC=1. Battery-backed, read/write.
C2P CNT2 detects falling edges when C2P = 0, rising edges when C2P = 1. C2P is “don’t care” when CC=1. The value
of Event Counter 2 may inadvertently increment if C2P is changed. Battery-backed, read/write.
C1P CNT1 detects falling edges when C1P = 0, rising edges when C1P = 1. The value of Event Counter 1 may
inadvertently increment if C1P is changed. Battery-backed, read/write.
0Bh Companion Control
D7 D6 D5 D4 D3 D2 D1 D0
SNL - FC WP1 WP0 VBC - VTP
SNL Serial Number Lock. Setting to a 1 makes registers 11h to 18h and SNL permanently read-only. SNL cannot be
cleared once set to 1. Nonvolatile, read/write.
FC Fast Charge: Setting FC to ‘1’ (and VBC=1) causes a ~1 mA trickle charge current to be supplied on VBAK.
Clearing VBC to ‘0’ disables the charge current. Nonvolatile, read/write.
WP1-0 Write Protect. These bits control the write protection of the memory array. Nonvolatile, read/write.
Write protect addresses WP1 WP0
None 0 0
Bottom 1/4 0 1
Bottom 1/2 1 0
Full array 1 1
VBC VBAK Charger Control. Setting VBC to ‘1’ (and FC=0) causes a ~80 µA (1 mA if FC=1) trickle charge current to
be supplied on VBA
K
. Clearing VBC to ‘0’ disables the charge current. Nonvolatile, read/write.
VTP VTP select. This bit controls the reset trip point for the low VDD reset function. Nonvolatile, read/write.
Trip Voltage VTP
2.6V 0
2.9V 1
0Ah Watchdog Control
D7 D6 D5 D4 D3 D2 D1 D0
WDE - - WDT4 WDT3 WDT2 WDT1 WDT0
WDE Watchdog Enable. When WDE=1, a watchdog timer fault will cause the /RST signal to go active. When WDE = 0
the timer runs but has no effect on /RST, however the WTR flag will be set when a fault occurs. Note as the timer
is free-running, users should restart the timer using WR3-0 prior to setting WDE=1. This assures a full watchdog
timeout interval occurs. Nonvolatile, read/write.
WDT4-0 Watchdog Timeout. Indicates the minimum watchdog timeout interval with 100 ms resolution. New watchdog
timeouts are loaded when the timer is restarted by writing the 1010b pattern to WR3-0. Nonvolatile, read/write.
Watchdog timeout WDT4 WDT3 WDT2 WDT1 WDT0
Invalid – default 100 ms 0 0 0 0 0
100 ms 0 0 0 0 1
200 ms 0 0 0 1 0
300 ms 0 0 0 1 1
.
.
.
2000 ms 1 0 1 0 0
2100 ms 1 0 1 0 1
2200 ms 1 0 1 1 0
.
.
.
2900 ms 1 1 1 0 1
3000 ms 1 1 1 1 0
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 14 of 26
Disable counter 1 1 1 1 1
09h Watchdog Restart & Flags
D7 D6 D5 D4 D3 D2 D1 D0
WTR POR LB - WR3 WR2 WR1 WR0
WTR Watchdog Timer Reset Flag: When a watchdog timer fault occurs, the WTR bit will be set to 1. It must be cleared
by the user. Note that both WTR and POR could be set if both reset sources have occurred since the flags were
cleared by the user. Battery-backed. Read/Write (internally set, user can clear bit).
POR Power-on Reset Flag: When the /RST pin is activated by VDD < VTP, the POR bit will be set to 1. It must be
cleared by the user. Note that both WTR and POR could be set if both reset sources have occurred since the flags
were cleared by the user. Battery-backed. Read/Write (internally set, user can clear bit).
LB Low Backup Flag: On power up, if the VBAK source is below the minimum voltage to operate the RTC and event
counters, this bit will be set to 1. The user should clear it to 0 when initializing the system. Battery-backed.
Read/Write (internally set, user can clear bit).
WR3-0 Watchdog Restart: Writing a pattern 1010b to WR3-0 restarts the watchdog timer. The upper nibble contents do
not affect this operation. Writing any pattern other than 1010b to WR3-0 has no effect on the timer. This allows
users to clear the WTR, POR, and LB flags without affecting the watchdog timer. Battery-backed, Write-only.
08h Timekeeping – Years
D7 D6 D5 D4 D3 D2 D1 D0
10 year.3 10 year.2 10 year.1 10 year.0 Year.3 Year.2 Year.1 Year.0
Contains the lower two BCD digits of the year. Lower nibble contains the value for years; upper nibble contains
the value for 10s of years. Each nibble operates from 0 to 9. The range for the register is 0-99. Battery-backed,
read/write.
07h Timekeeping – Months
D7 D6 D5 D4 D3 D2 D1 D0
0 0 0
10 Month Month.3 Month.2 Month.1 Month.0
Contains the BCD digits for the month. Lower nibble contains the lower digit and operates from 0 to 9; upper
nibble (one bit) contains the upper digit and operates from 0 to 1. The range for the register is 1-12. Battery-
backed, read/write.
06h Timekeeping – Date of the month
D7 D6 D5 D4 D3 D2 D1 D0
0 0
10 date.1 10 date.0 Date.3 Date.2 Date.1 Date.0
Contains the BCD digits for the date of the month. Lower nibble contains the lower digit and operates from 0 to 9;
upper nibble contains the upper digit and operates from 0 to 3. The range for the register is 1-31. Battery-backed,
read/write.
05h Timekeeping – Day of the week
D7 D6 D5 D4 D3 D2 D1 D0
0 0 0 0 0 Day.2 Day.1 Day.0
Lower nibble contains a value that correlates to day of the week. Day of the week is a ring counter that counts
from 1 to 7 then returns to 1. The user must assign meaning to the day value, as the day is not integrated with the
date. Battery-backed, read/write.
04h Timekeeping – Hours
D7 D6 D5 D4 D3 D2 D1 D0
0 0
10 hours.1 10 hours.0 Hours.3 Hours2 Hours.1 Hours.0
Contains the BCD value of hours in 24-hour format. Lower nibble contains the lower digit and operates from 0 to
9; upper nibble (two bits) contains the upper digit and operates from 0 to 2. The range for the register is 0-23.
Battery-backed, read/write.
03h Timekeeping – Minutes
D7 D6 D5 D4 D3 D2 D1 D0
0 10 min.2 10 min.1 10 min.0 Min.3 Min.2 Min.1 Min.0
Contains the BCD value of minutes. Lower nibble contains the lower digit and operates from 0 to 9; upper nibble
contains the upper minutes digit and operates from 0 to 5. The range for the register is 0-59. Battery-backed,
read/write.
02h Timekeeping – Seconds
D7 D6 D5 D4 D3 D2 D1 D0
0 10 sec.2 10 sec.1 10 sec.0 Seconds.3 Seconds.2 Seconds.1 Seconds.0
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 15 of 26
Contains the BCD value of seconds. Lower nibble contains the lower digit and operates from 0 to 9; upper nibble
contains the upper digit and operates from 0 to 5. The range for the register is 0-59. Battery-backed, read/write.
01h CAL/Control
D7 D6 D5 D4 D3 D2 D1 D0
OSCEN Reserved CALS CAL.4 CAL.3 CAL.2 CAL.1 CAL.0
/OSCEN /Oscillator Enable. When set to 1, the oscillator is halted. When set to 0, the oscillator runs. Disabling the
oscillator can save battery power during storage. On a power-up without battery, this bit is set to 1. Battery-
backed, read/write.
Reserved Reserved bits. Do not use. Should remain set to 0.
CALS Calibration sign. Determines if the calibration adjustment is applied as an addition to or as a subtraction from
the time-base. Calibration is explained on page 7. Nonvolatile, read/write.
CAL.4-0 These five bits control the calibration of the clock. Nonvolatile, read/write.
00h Flags/Control
D7 D6 D5 D4 D3 D2 D1 D0
Reserved CF Reserved Reserved Reserved CAL W R
CF Century Overflow Flag. This bit is set to a 1 when the values in the years register overflows from 99 to 00. This
indicates a new century, such as going from 1999 to 2000 or 2099 to 2100. The user should record the new
century information as needed. This bit is cleared to 0 when the Flag register is read. It is read-only for the user.
Battery-backed.
CAL Calibration Mode. When set to 1, the clock enters calibration mode. When CAL is set to 0, the clock operates
normally, and the CAL/PFO pin is controlled by the power fail comparator. Battery-backed, read/write.
W Write Time. Setting the W bit to 1 freezes the clock. The user can then write the timekeeping registers with
updated values. Resetting the W bit to 0 causes the contents of the time registers to be transferred to the
timekeeping counters and restarts the clock. Battery-backed, read/write.
R Read Time. Setting the R bit to 1 copies a static image of the timekeeping core and place it into the user
registers. The user can then read them without concerns over changing values causing system errors. The R bit
going from 0 to 1 causes the timekeeping capture, so the bit must be returned to 0 prior to reading again.
Battery-backed, read/write.
Reserved Reserved bits. Do not use. Should remain set to 0.
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 16 of 26
Two-wire Interface
The FM31L27x employs an industry standard two-
wire bus that is familiar to many users. This product
is unique since it incorporates two logical devices in
one chip. Each logical device can be accessed
individually. Although monolithic, it appears to the
system software to be two separate products. One is
a memory device. It has a Slave Address (Slave ID =
1010b) that operates the same as a stand-alone
memory device. The second device is a real-time
clock and processor companion which have a unique
Slave Address (Slave ID = 1101b).
By convention, any device that is sending data onto
the bus is the transmitter while the target device for
this data is the receiver. The device that is
controlling the bus is the master. The master is
responsible for generating the clock signal for all
operations. Any device on the bus that is being
controlled is a slave. The FM31L27x is always a
slave device.
The bus protocol is controlled by transition states in
the SDA and SCL signals. There are four conditions:
Start, Stop, Data bit, and Acknowledge. The figure
below illustrates the signal conditions that specify
the four states. Detailed timing diagrams are shown
in the Electrical Specifications section.
Stop
(Master) Start
(Master)
7
Data bits
(Transmitter)
60
Data bit
(Transmitter) Acknowledge
(Receiver)
SCL
SDA
Figure 11. Data Transfer P rotocol
Start Condition
A Start condition is indicated when the bus master
drives SDA from high to low while the SCL signal is
high. All read and write transactions begin with a
Start condition. An operation in progress can be
aborted by asserting a Start condition at any time.
Aborting an operation using the Start condition will
ready the FM31L27x for a new operation.
If the power supply drops below the specified VTP
during operation, any 2-wire transaction in progress
will be aborted and the system must issue a Start
condition prior to performing another operation.
Stop Condition
A Stop condition is indicated when the bus master
drives SDA from low to high while the SCL signal is
high. All operations must end with a Stop condition.
If an operation is pending when a stop is asserted,
the operation will be aborted. The master must have
control of SDA (not a memory read) in order to
assert a Stop condition.
Data/Address Transfer
All data transfers (including addresses) take place
while the SCL signal is high. Except under the two
conditions described above, the SDA signal should
not change while SCL is high.
Acknowledge
The Acknowledge (ACK) takes place after the 8th
data bit has been transferred in any transaction.
During this state the transmitter must release the
SDA bus to allow the receiver to drive it. The
receiver drives the SDA signal low to acknowledge
receipt of the byte. If the receiver does not drive
SDA low, the condition is a No-Acknowledge
(NACK) and the operation is aborted.
The receiver might NACK for two distinct reasons.
First is that a byte transfer fails. In this case, the
NACK ends the current operation so that the part can
be addressed again. This allows the last byte to be
recovered in the event of a communication error.
Second and most common, the receiver does not
send an ACK to deliberately terminate an operation.
For example, during a read operation, the FM31L27x
will continue to place data onto the bus as long as the
receiver sends ACKs (and clocks). When a read
operation is complete and no more data is needed,
the receiver must NACK the last byte. If the receiver
ACKs the last byte, this will cause the FM31L27x to
attempt to drive the bus on the next clock while the
master is sending a new command such as a Stop.
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
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Slave Address
The first byte that the FM31L27x expects after a
Start condition is the slave address. As shown in
figures below, the slave address contains the Slave
ID, Device Select address, and a bit that specifies if
the transaction is a read or a write.
The FM31L27x has two Slave Addresses (Slave IDs)
associated with two logical devices. To access the
memory device, bits 7-4 should be set to 1010b. The
other logical device within the FM31L27x is the
real-time clock and companion. To access this
device, bits 7-4 of the slave address should be set to
1101b. A bus transaction with this slave address will
not affect the memory in any way. The figures
below illustrate the two Slave Addresses.
The Device Select bits allow multiple devices of the
same type to reside on the 2-wire bus. The device
select bits (bits 2-1) select one of four parts on a two-
wire bus. They must match the corresponding value
on the external address pins in order to select the
device. Bit 0 is the read/write bit. A “1” indicates a
read operation, and a “0” indicates a write operation.
1 0 1 0 X A1 A0 R/W
Slave ID Device
Select
76543 21 0
Figure 12. Slave Address - Memory
Figure 13. Slave Address – Companion
Addressing Overview – Memory
After the FM31L27x acknowledges the Slave
Address, the master can place the memory address
on the bus for a write operation. The address requires
two bytes. This is true for all members of the family.
The first is the MSB (upper byte). For a given
density unused address bits are don’t cares, but
should be set to 0 to maintain upward compatibility.
Following the MSB is the LSB (lower byte) which
contains the remaining eight address bits. The
address is latched internally. Each access causes the
latched address to be incremented automatically. The
current address is the value that is held in the latch,
either a newly written value or the address following
the last access. The current address will be held as
long as VDD > VTP or until a new value is written.
Accesses to the clock do not affect the current
memory address. Reads always use the current
address. A random read address can be loaded by
beginning a write operation as explained below.
After transmission of each data byte, just prior to the
Acknowledge, the FM31L27x increments the
internal address. This allows the next sequential byte
to be accessed with no additional addressing
externally. After the last address is reached, the
address latch will roll over to 0000h. There is no
limit to the number of bytes that can be accessed
with a single read or write operation.
Addressing Overview – RTC & Companion
The RTC and Processor Companion operate in a
similar manner to the memory, except that it uses
only one byte of address. Addresses 00h to 18h
correspond to special function registers. Attempting
to load addresses above 18h is an illegal condition;
the FM31L27x will return a NACK and abort the 2-
wire transaction.
Data Transfer
After the address information has been transmitted,
data transfer between the bus master and the
FM31L27x begins. For a read, the FM31L27x will
place 8 data bits on the bus then wait for an ACK
from the master. If the ACK occurs, the FM31L27x
will transfer the next byte. If the ACK is not sent, the
FM31L27x will end the read operation. For a write
operation, the FM31L27x will accept 8 data bits
from the master then send an Acknowledge. All data
transfer occurs MSB (most significant bit) first.
Memory Write Operation
All memory writes begin with a Slave Address, then
a memory address. The bus master indicates a write
operation by setting the slave address LSB to a 0.
After addressing, the bus master sends each byte of
data to the memory and the memory generates an
Acknowledge condition. Any number of sequential
bytes may be written. If the end of the address range
is reached internally, the address counter will wrap
to 0000h. Internally, the actual memory write occurs
after the 8th data bit is transferred. It will be complete
before the Acknowledge is sent. Therefore, if the
user desires to abort a write without altering the
memory contents, this should be done using a Start
or Stop condition prior to the 8th data bit. The figures
11 0 1 X
A
1
A
0 R/W
Slave ID
76543 21 0
Device
Select
FM31L278/ L276 - 3V I2C Companion
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standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
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Rev. 3.0
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below illustrate a single- and multiple-writes to memory.
Figure 14. Single Byte Memory Write
Figure 15. Multiple Byte Memory Write
Memory Read Operation
There are two types of memory read operations. They
are current address read and selective address read. In
a current address read, the FM31L27x uses the
internal address latch to supply the address. In a
selective read, the user performs a procedure to first
set the address to a specific value.
Current Address & Sequential Read
As mentioned above the FM31L27x uses an internal
latch to supply the address for a read operation. A
current address read uses the existing value in the
address latch as a starting place for the read
operation. The system reads from the address
immediately following that of the last operation.
To perform a current address read, the bus master
supplies a slave address with the LSB set to 1. This
indicates that a read operation is requested. After
receiving the complete device address, the
FM31L27x will begin shifting data out from the
current address on the next clock. The current address
is the value held in the internal address latch.
Beginning with the current address, the bus master
can read any number of bytes. Thus, a sequential read
is simply a current address read with multiple byte
transfers. After each byte the internal address counter
will be incremented.
Each time the bus master acknowledges a byte,
this indicates that the FM31L27x should read
out the next sequential byte.
There are four ways to terminate a read operation.
Failing to properly terminate the read will most likely
create a bus contention as the FM31L27x attempts to
read out additional data onto the bus. The four valid
methods follow.
1. The bus master issues a NACK in the 9th clock
cycle and a Stop in the 10th clock cycle. This is
illustrated in the diagrams below and is
preferred.
2. The bus master issues a NACK in the 9th clock
cycle and a Start in the 10th.
3. The bus master issues a Stop in the 9th clock
cycle.
4. The bus master issues a Start in the 9th clock
cycle.
If the internal address reaches the top of memory, it
will wrap around to 0000h on the next read cycle.
The figures below show the proper operation for
current address reads.
Selective (Random) Read
There is a simple technique that allows a user to
select a random address location as the starting point
for a read operation. This involves using the first
three bytes of a write operation to set the internal
address followed by subsequent read operations.
S
A
Slave Address 0
A
ddress MSB
A
Data Byte
A
P
By Master
By FM31L27x
Start
A
ddress & Data Stop
A
cknowledge
A
ddress LSB
A
Data Byte
A
S
A
Slave Address 0 Address MSB
A
Data Byte
A
P
By Master
By FM31L27x
Start
A
ddress & Data Stop
A
cknowledge
A
ddress LSB
A
FM31L278/ L276 - 3V I2C Companion
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standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
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To perform a selective read, the bus master sends out
the slave address with the LSB set to 0. This specifies
a write operation. According to the write protocol,
the bus master then sends the address bytes that are
loaded into the internal address latch. After the
FM31L27x acknowledges the address, the bus master
issues a Start condition. This simultaneously aborts
the write operation and allows the read command to
be issued with the slave address LSB set to a 1. The
operation is now a read from the current address.
Read operations are illustrated below.
RTC/Companion Write Operation
All RTC and Companion writes operate in a similar
manner to memory writes. The distinction is that a
different device ID is used and only one byte address
is needed instead of two. Figure 16 illustrates a single
byte write to this device.
RTC/Companion Read Operation
As with writes, a read operation begins with the
Slave Address. To perform a register read, the bus
master supplies a Slave Address with the LSB set to
1. This indicates that a read operation is requested.
After receiving the complete Slave Address, the
FM31L27x will begin shifting data out from the
current register address on the next clock. Auto-
increment operates for the special function registers
as with the memory address. A current address read
for the registers look exactly like the memory except
that the device ID is different.
The FM31L27x contains two separate address
registers, one for the memory address and the other
for the register address. This allows the contents of
one address register to be modified without affecting
the current address of the other register. For example,
this would allow an interrupted read to the memory
while still providing fast access to an RTC register. A
subsequent memory read will then continue from the
memory address where it previously left off, without
requiring the load of a new memory address.
However, a write sequence always requires an
address to be supplied.
Figure 16. Current Address Memory Read
Figure 17. Sequential Memory Read
S
A
Slave Address 1Data Byte 1 P
By Master
By FM31L27x
Start
A
ddress Stop
A
cknowledge
No
A
cknowledge
Data
Data Byte
A
A
cknowledge
S
A
Slave Address 1Data Byte 1 P
By Master
By FM31L27x
Start
A
ddress Stop
A
cknowledge
No
A
cknowledge
Data
FM31L278/ L276 - 3V I2C Companion
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standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
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Figure 18. Selective (Random) Memory Read
Figure 19. Byte Register Write
(Although not required, it is recommended that A5-A7 in the Register Address byte are zeros in order
to preserve compatibility with future devices.)
Addressing F-RAM Array in the FM31L27x Family
The FM31L27x family includes 256Kb and 64Kb memory densities. The following 2-byte address field is shown for
each density.
Table 4. Two-Byte Memory Address
Part # 1s
t
Address Byte 2n
d
Address Byte
FM31L278 x A14 A13 A12 A11 A10 A9 A8 A7 A6 A5 A4 A3 A2 A1 A0
FM31L276 x x x A12 A11 A10 A9 A8
A7 A6 A5 A4 A3 A2 A1 A0
S
A
Slave Address 0
A
ddress
A
Data Byte
A
P
By Master Start
A
ddress & Data Stop
A
cknowledge
000
By FM31L27x
S
A
Slave Address 1Data Byte 1 P
By Master
By FM31L27x
Start
A
ddress Stop
No
A
cknowledge
Data
S
A
Slave Address 0
A
ddress MSB
A
Start
A
ddress
A
cknowledge
A
ddress LSB
A
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
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Electrical Specifications
Absolute Maximum Ratings
Symbol Description Ratings
VDD Power Supply Voltage with respect to VSS -1.0V to +5.0V
VIN Voltage on any signal pin with respect to VSS -1.0V to +5.0V * and
VIN ≤ VDD+1.0V **
VBA
K
Backup Supply Voltage -1.0V to +4.5V
TSTG Storage Temperature -55°C to + 125°C
TLEAD Lead Temperature (Soldering, 10 seconds) 260° C
VESD Electrostatic Discharge Voltage
- Human Body Model (AEC-Q100-002 Rev. D)
- Charged Device Model (AEC-Q100-011 Rev. B)
- Machine Model (AEC-Q100-003 Rev. E)
2kV
1.25kV
100V
Package Moisture Sensitivity Level MSL-2
* PFI input voltage must not exceed 4.5V.
** The “VIN < VDD+1.0V” restriction does not apply to the SCL and SDA inputs which do not employ a diode to VDD.
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only,
and the functional operation of the device at these or any other conditions above those listed in the operational section of this
specification is not implied. Exposure to absolute maximum ratings conditions for extended periods may affect device reliability.
DC Operating Conditions (TA = -40° C to + 85° C, VDD = 2.7V to 3.6V unless otherwise specified)
Symbol Parameter Min Typ Max Units Notes
VDD Main Power Supply 2.7 3.6 V 7
IDD V
DD Supply Current
@ SCL = 100 kHz
@ SCL = 400 kHz
@ SCL = 1 MHz
500
900
1500
μA
μA
μA
1
ISB Standby Current
120
μ
A 2
VBAK RTC Backup Supply Voltage
@ TA = +25ºC to +85ºC
@ T
A
= -40ºC to +25ºC
1.55
1.9
3.75
3.75
V
V
V
9
IBAK RTC Backup Supply Current
@ TA = +25ºC, VBAK = 3.0V
@ TA = +85ºC, VBAK = 3.0V
@ TA = +25ºC, VBAK = 2.0V
@ TA = +85ºC, VBAK = 2.0V
1.4
2.1
1.15
1.75
μA
μA
μA
μA
4
IBAKTC Trickle Charge Current with VBAK=0V
Fast Charge Off (FC = 0)
Fast Charge On (FC = 1)
50
200
-
-
120
2500
μA
μA
10
VTP1 V
DD Trip Point Voltage, VTP = 0 2.55 2.6 2.70 V 5
VTP2 V
DD Trip Point Voltage, VTP = 1 2.85 2.9 3.00 V 5
VRST VDD for valid /RST @ IOL = 80
μ
A at VOL
VBAK > VBAK min
VBA
K
< VBA
K
min
0
1.6
V
V
6
ILI Input Leakage Current ±1
μ
A 3
ILO Output Leakage Current ±1
μ
A 3
VIL Input Low Voltage
All inputs except those listed below
CNT1-2 battery backed (VDD < 2.5V)
CNT1-2 (VDD > 2.5V)
-0.3
-0.3
-0.3
0.3 VDD
0.5
0.8
V
V
V
8
VIH Input High Voltage
All inputs except those listed below
PFI (comparator input)
CNT1-2 battery backed (VDD < 2.5V)
CNT1-2 VDD > 2.5V
0.7 VDD
-
VBAK – 0.5
0.7 VDD
VDD + 0.3
3.75
VBAK + 0.3
VDD + 0.3
V
V
V
V
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
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Rev. 3.0
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DC Operating Conditions, continued (TA = -40° C to + 85° C, VDD = 2.7V to 3.6V unless otherwise specified)
Symbol Parameter Min Typ Max Units
Notes
VOL Output Low Voltage (IOL = 3 mA) - 0.4 V
VOH Output High Voltage (IOH = -2 mA) 2.4 - V
RRST Pull-up Resistance for /RST Inactive 50 400 KΩ
RIN Input Resistance (pulldown)
A1-A0 for VIN = VIL max
A1-A0 for VIN = VIH min
20
1
KΩ
MΩ
VPFI Power Fail Input Reference Voltage 1.175 1.20 1.225 V
VHYS Power Fail Input (PFI) Hysteresis (Rising) - 100 mV
Notes
1. SCL toggling between VDD-0.3V and VSS, other inputs VSS or VDD-0.3V.
2. All inputs at VSS or VDD, static. Stop command issued.
3. VIN or VOUT = VSS to VDD. Does not apply to A0, A1, or /RST pins.
4. VBAK = 3.0V, VDD < 2.4V, oscillator running, CNT1-2 at VBAK.
5. /RST is asserted low when VDD < VTP.
6. The minimum VDD to guarantee the level of /RST remains a valid VOL level.
7. Full complete operation. Supervisory circuits, RTC, etc operate to lower voltages as specified.
8. Includes /RST input detection of external reset condition to trigger driving of /RST signal by FM31L27x.
9. The VBAK trickle charger automatically regulates the maximum voltage on this pin for capacitor backup applications.
10. VBAK will source current when trickle charge is enabled (VBC bit=1), VDD > VBAK, and VBAK < VBAK max.
AC Parameters (TA = -40° C to + 85° C, VDD = 2.7V to 3.6V, CL = 100 pF unless otherwise specified)
Symbol Parameter Min Max Min Max Min Max Units Notes
fSCL SCL Clock Frequency 0 100 0 400 0 1000 kHz
tLOW Clock Low Period 4.7 1.3 0.6
μ
s
tHIGH Clock High Period 4.0 0.6 0.4
μ
s
tAA SCL Low to SDA Data Out Valid 3 0.9 0.55
μ
s
tBUF Bus Free Before New Transmission 4.7 1.3 0.5
μ
s
tHD:STA Start Condition Hold Time 4.0 0.6 0.25
μ
s
tSU:STA Start Condition Setup for Repeated
Start
4.7 0.6 0.25
μ
s
tHD:DAT Data In Hold Time 0 0 0 ns
tSU:DAT Data In Setup Time 250 100 100 ns
tR Input Rise Time 1000 300 300 ns 1
tF Input Fall Time 300 300 100 ns 1
tSU:STO Stop Condition Setup Time 4.0 0.6 0.25
μ
s
tDH Data Output Hold (from SCL @ VIL) 0 0 0 ns
tSP Noise Suppression Time Constant on
SCL, SDA
50 50 50 ns
All SCL specifications as well as start and stop conditions apply to both read and write operations.
Capacitance (TA = 25° C, f=1.0 MHz, VDD = 3.0V)
Symbol Parameter Typ Max Units Notes
CIO Input/Output Capacitance - 8 pF 1
CXTAL X1, X2 Crystal pin Capacitance 25 - pF 1, 2
Notes
1 This parameter is characterized but not tested.
2 The crystal attached to the X1/X2 pins must be rated as 12.5pF.
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 23 of 26
Supervisor Timing (TA = -40° C to + 85° C, VDD = 2.7V to 3.6V)
Symbol Parameter Min Max Units Notes
tRPU /RST Active (low) after VDD>VTP 100 200 ms
tRNR V
DD < VTP noise immunity 10 25
μ
s 1
tVR V
DD Rise Time 50 -
μ
s/V 1,2
tVF V
DD Fall Time 100 -
μ
s/V 1,2
tWDP Pulse Width of /RST for Watchdog Reset 100 200 ms
tWDOG Timeout of Watchdog tDOG 2*tDOG ms 3
fCNT Frequency of Event Counters 0 10 MHz
Notes
1 This parameter is characterized but not tested.
2 Slope measured at any point on VDD waveform.
3 tDOG is the programmed time in register 0Ah, VDD > VTP and tRPU satisfied.
/RST Timing
Data Retention (VDD = 2.7V to 3.6V)
Symbol Parameter Min Units Notes
TDR Data Retention
@ +75°C
@ +80°C
@ +85°C
38
19
10
Years
Years
Years
VDD
VTP
VRST
RST
tRPU
tRNR
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
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AC Test Conditions Equivalent AC Load Circuit
Input Pulse Levels 0.1 VDD to 0.9 VDD
Input rise and fall times 10 ns
Input and output timing levels 0.5 VDD
Diagram Notes
All start and stop timing parameters apply to both read and write
cycles. Clock specifications are identical for read and write cycles.
Write timing parameters apply to slave address, word address, and
write data bits. Functional relationships are illustrated in the relevant
data sheet sections. These diagrams illustrate the timing parameters
only.
Read Bus Timing
tSU:STA
Start
tR`tF
Stop Start
tBUF
tHIGH
1/fSCL
tLOW tSP tSP
Acknowledge
tHD:DAT
tSU:DAT
tAA tDH
SCL
SDA
Write Bus Timing
tSU:STO
Start Stop Start Acknowledge
tAA
tHD:DAT
tHD:STA tSU:DAT
SCL
SDA
3.6V
Output
1066 Ω
100 pF
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 25 of 26
Mechanical Drawing
14-pin SOIC (JEDEC Standard MS-012, Variation AB)
Pin 1
3.90 ±0.136.00 ±0.20
8.64 ±0.10
0.10
0.25
1.35
1.75
0.33
0.51
1.27 0.10 mm
0.25
0.50 45°
0.40
1.27
0.19
0.25
0°-8°
Recommended PCB Footprint
7.70
0.65
1.27
2.00
3.70
. . .
. . .
Refer to JEDEC MS-012 for complete dimensions and notes.
All dimensions in millimeters.
SOIC Package Marking Scheme
Legend:
XXXXXX= part number, P= package type (G=”Green”/RoHS)
R=rev, LLLLLLL= lot code
RIC=Ramtron Int’l Corp, YY=year, WW=work week
Example: FM31L278, “Green” SOIC package, Year 2011, Work Week 06
FM31L278-G
D70023G
RIC 1106
XXXXXXX-P
RLLLLLLL
RIC YYWW
FM31L278/ L276 - 3V I2C Companion
This product conforms to specifications per the terms of the Ramtron Ramtron International Corporation
standard warranty. The product has completed Ramtron’s internal 1850 Ramtron Drive, Colorado Springs, CO 80921
qualification testing and has reached production status. (800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012 Page 26 of 26
Revision History
Revision
Date
Summary
2.0 1/31/2011 Pre-Production. Rev D. Changed IBAK and VBAK specs. Added curves to
Backup Power section (p.7).
2.1 9/12/2011 FM31L274 and FM31L272 are End of Life (EOL). There are no direct
replacements for these parts. The FM31L278 and FM31L276 are unaffected
by this change.
3.0 8/14/2012 Modified formatting, deleted 4Kb and 8Kb versions and changed to
production status
