Pre-Production
This is a product in pre-production phase of development. Device Ramtron International Corporation
characterization is complete and Ramtron does not expect to change 1850 Ramtron Drive, Colorado Springs, CO 80921
the specifications. Ramtron will issue a Product Change Notice if (800) 545-FRAM, (719) 481-7000
any specification changes are made. http://www.ramtron.com
Rev. 2.1
Sept. 2011 Page 1 of 26
FM31278/FM31276
FM31274/FM31272
5V Integrated Processor Companion with Memory
Features
High Integration Device Replaces Multiple 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
4Kb, 16Kb, 64Kb, and 256Kb versions
Virtually Unlimited Read/Write Endurance
38 year Data Retention (+75C)
NoDelay™ Writes
Real-time Clock/Calendar
Backup Current under 1.4 A
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 4.0 to 5.5V
14-pin “Green”/RoHS SOIC package (-G)
Low Operating Current
-40C to +85C Operation
Underwriters Laboratory (UL) Recognized
Description
The FM3127x 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 4.0 to 5.5V.
Each FM3127x provides nonvolatile RAM available
in sizes including 4Kb, 16Kb, 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.
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 2 of 26
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.
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
FM31278
4.0-5.5V
3.9, 4.4V
FM31278-G
4.0-5.5V
3.9, 4.4V
FM31278-GTR (tape&reel)
FM31276
4.0-5.5V
3.9, 4.4V
FM31276-G
4.0-5.5V
3.9, 4.4V
FM31276-GTR (tape&reel)
FM31274 *
4.0-5.5V
3.9, 4.4V
FM31274-G
4.0-5.5V
3.9, 4.4V
FM31274-GTR (tape&reel)
FM31272 *
4.0-5.5V
3.9, 4.4V
FM31272-G
4.0-5.5V
3.9, 4.4V
FM31272-GTR (tape&reel)
* The FM31274 and FM31272 devices are EOL as of Sept. 2011.
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
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 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 Registers
Event
Counters
CNT1
CNT2
Special
Function
Registers
S/N
X1
X2
LockOut
LockOut
+
-
1.2V
Watchdog
LV Detect
Switched Power
512Hz
RTC Cal.
Battery Backed
Nonvolatile
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 4 of 26
Overview
The FM3127x 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 FM3127x 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 FM3127x is a family of products available in
different memory sizes including 4Kb, 16Kb, 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 ¼ 0 1
Bottom ½ 1 0
Full array 1 1
Processor Companion
In addition to nonvolatile RAM, the FM3127x 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 FM3127x 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
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”.
VDD
VTP
tRPU
RST
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 5 of 26
VTP VTP
3.9V 0
4.4V 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
FM3127x 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.
Figure 5. Comparator as Early Power-Fail Warning
+
-
1.2V ref
Regulator
VDD
FM3127x
To MCU
NMI input
CAL/PFO
PFI
Timebase
Counter
Watchdog
timeout
100 ms
clock
WDE
/RST
WR3-0
= 1010b to restart
FM3127x
Reset
Switch
RST
MCU
Switch
Behavior
RST
FM3127x
drives
100 ms (min.)
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 6 of 26
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 FM3127x 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 cleared.
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
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
16-bit Counter
CNT1
CC
CNT2
C1P
C2P
16-bit Counter
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 7 of 26
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 Voltage
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
3.75V. In 5V systems, this charges the capacitor to
VDD without an external diode and resistor charger
and also prevents the user from exceeding the VBAK
maximum voltage specification. 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 source is used, the VBAK
pin should be tied to VSS. VBAK should not be tied to
5V since the VBAK (max) specification will be
exceeded. 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
series
resistor as a safety element. The trickle charger is UL
Recognized.
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 8 of 26
32.768 kHz
crystal
Oscillator
Clock
Divider
Update
Logic
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 FM3127x 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 FM3127x. 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
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.
X1 X2
Vdd
FM3127x
R1
R2
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 9 of 26
Layout for Surface Mount Crystal Layout for Through Hole Crystal
(red = top layer, green = bottom layer) (red = top layer, green = bottom layer)
VDD
SCL
SDA
X2
X1
PFI
VBAK
VDD
SCL
SDA
X2
X1
PFI
VBAK
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 10 of 26
Calibration Adjustments
Positive Calibration for slow clocks: Calibration will achieve 2.17 PPM after 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
Negative Calibration for fast clocks: Calibration will achieve 2.17 PPM after 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
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 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 - VTP 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 0 0 0 10 mo months Month 1-12
06h 0 0 10 date date Date 1-31
05h 0 0 0 0 0 day Day 1-7
04h 0 0 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
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 12 of 26
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.
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 13 of 26
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 ¼ 0 1
Bottom ½ 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 VBAK. 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
3.9V 0
4.4V 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
Disable counter 1 1 1 1 1
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 14 of 26
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
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.
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 15 of 26
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.
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 16 of 26
Two-wire Interface
The FM3127x 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 FM3127x 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)
6 0
Data bit
(Transmitter)Acknowledge
(Receiver)
SCL
SDA
Figure 11. Data Transfer Protocol
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 FM3127x 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 FM3127x
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 FM3127x to
attempt to drive the bus on the next clock while the
master is sending a new command such as a Stop.
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 17 of 26
Slave Address
The first byte that the FM3127x 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 FM3127x 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 FM3127x 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
7 6 5 4 3 2 1 0
Figure 12. Slave Address - Memory
Figure 13. Slave Address – Companion
Addressing Overview – Memory
After the FM3127x 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.
Therefore the 4Kb and 16Kb configurations will be
addressed differently from stand alone serial
memories but the entire family will be upwardly
compatible with no software changes.
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 FM3127x 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 FM3127x 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
FM3127x begins. For a read, the FM3127x will
place 8 data bits on the bus then wait for an ACK
from the master. If the ACK occurs, the FM3127x
will transfer the next byte. If the ACK is not sent, the
FM3127x will end the read operation. For a write
operation, the FM3127x 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
1
1
0
1
X
A1
A0
R/W
Slave ID
7
6
5
4
3
2
1
0
Device
Select
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 18 of 26
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
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 FM3127x 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 FM3127x 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 FM3127x
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 FM3127x 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 FM3127x 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
S
A
Slave Address
0
Address MSB
A
Data Byte
A
P
By Master
By FM3127x
Start
Address & Data
Stop
Acknowledge
Address LSB
A
Data Byte
A
S
A
Slave Address
0
Address MSB
A
Data Byte
A
P
By Master
By FM3127x
Start
Address & Data
Stop
Acknowledge
Address LSB
A
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 19 of 26
three bytes of a write operation to set the internal
address followed by subsequent read operations.
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
FM3127x 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
FM3127x 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 FM3127x 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
1
Data Byte
1
P
By Master
By FM3127x
Start
Address
Stop
Acknowledge
No
Acknowledge
Data
Data Byte
A
Acknowledge
S
A
Slave Address
1
Data Byte
1
P
By Master
By FM3127x
Start
Address
Stop
Acknowledge
No
Acknowledge
Data
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 20 of 26
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 FM3127x Family
The FM3127x family includes 256Kb, 64Kb, 16Kb, and 4Kb memory densities. The following 2-byte address field
is shown for each density.
Table 4. Two-Byte Memory Address
Part #
1st Address Byte
2nd Address Byte
FM31278
x
A14
A13
A12
A11
A10
A9
A8
A7
A6
A5
A4
A3
A2
A1
A0
FM31276
x
x
x
A12
A11
A10
A9
A8
A7
A6
A5
A4
A3
A2
A1
A0
FM31274
x
x
x
x
x
A10
A9
A8
A7
A6
A5
A4
A3
A2
A1
A0
FM31272
x
x
x
x
x
x
x
A8
A7
A6
A5
A4
A3
A2
A1
A0
Note: The FM31274 and FM31272 are no longer available.
S
A
Slave Address
1
Data Byte
1
P
By Master
By FM3127x
Start
Address
Stop
No
Acknowledge
Data
S
A
Slave Address
0
Address MSB
A
Start
Address
Acknowledge
Address LSB
A
S
A
Slave Address
0
Address
A
Data Byte
A
P
By Master
Start
Address & Data
Stop
Acknowledge
0
0
0
By FM3127x
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 21 of 26
Electrical Specifications
Absolute Maximum Ratings
Symbol
Description
Ratings
VDD
Power Supply Voltage with respect to VSS
-1.0V to +7.0V
VIN
Voltage on any signal pin with respect to VSS
-1.0V to +7.0V * and
VIN ≤ VDD+1.0V **
VBAK
Backup Supply Voltage
-1.0V to +4.5V
TSTG
Storage Temperature
-55C to + 125C
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 = 4.0V to 5.5V unless otherwise specified)
Symbol
Parameter
Min
Typ
Max
Units
Notes
VDD
Main Power Supply
4.0
5.5
V
7
IDD
VDD Supply Current
@ SCL = 100 kHz
@ SCL = 400 kHz
@ SCL = 1 MHz
500
900
1500
A
A
A
1
ISB
Standby Current
150
A
2
VBAK
RTC Backup Supply Voltage
@ TA = +25ºC to +85ºC
@ TA = -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
A
A
A
A
4
IBAKTC
Trickle Charge Current with VBAK=0V
Fast Charge Off (FC = 0)
Fast Charge On (FC = 1)
50
200
-
-
120
2500
A
A
10
VTP1
VDD Trip Point Voltage, VTP = 0
3.80
3.9
4.00
V
5
VTP2
VDD Trip Point Voltage, VTP = 1
4.25
4.4
4.50
V
5
VRST
VDD for valid /RST @ IOL = 80 A at VOL
VBAK > VBAK min
VBAK < VBAK min
0
1.6
V
V
6
ILI
Input Leakage Current
1
A
3
ILO
Output Leakage Current
1
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
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 22 of 26
DC Operating Conditions, continued (TA = -40 C to + 85 C, VDD = 4.0V to 5.5V 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
K
RIN
Input Resistance (pulldown)
A1-A0 for VIN = VIL max
A1-A0 for VIN = VIH min
20
1
K
M
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 FM3127x.
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 = 4.0V to 5.5V, 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
s
tHIGH
Clock High Period
4.0
0.6
0.4
s
tAA
SCL Low to SDA Data Out Valid
3
0.9
0.55
s
tBUF
Bus Free Before New Transmission
4.7
1.3
0.5
s
tHD:STA
Start Condition Hold Time
4.0
0.6
0.25
s
tSU:STA
Start Condition Setup for Repeated
Start
4.7
0.6
0.25
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
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 = 5.0V)
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.
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 23 of 26
Supervisor Timing (TA = -40 C to + 85 C, VDD = 4.0V to 5.5V)
Symbol
Parameter
Min
Max
Units
Notes
tRPU
/RST Active (low) after VDD>VTP
100
200
ms
tRNR
VDD < VTP noise immunity
10
25
s
1
tVR
VDD Rise Time
50
-
s/V
1,2
tVF
VDD Fall Time
100
-
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 = 4.0V to 5.5V)
Symbol
Parameter
Min
Units
Notes
TDR
Data Retention
@ +75°C
@ +80°C
@ +85°C
38
19
10
Years
Years
Years
VDD
VTP
VRST
RST
t
RPU
t
RNR
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 Page 24 of 26
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
5.5V
Output
1700
100 pF
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 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: FM31278, “Green” SOIC package, Year 2011, Work Week 06
FM31278-G
D70023G1
RIC 1106
XXXXXXX-P
RLLLLLLL
RIC YYWW
FM31278/276/274/272 - 5V I2C Companion
Rev. 2.1
Sept. 2011 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
FM31274 and FM31272 are End of Life (EOL). There are no direct
replacements for these parts. The FM31278 and FM31276 are unaffected
by this change.