AN-405
http://www.ramtron.com Ramtron International Corporation
1850 Ramtron Drive, Colorado Springs, CO 80921
(800) 54 5- FRA M, (7 19) 481- 7000
Dec. 20 07 Page 1 of 2
Comparing FM31xx and FM3127x/FM31L27x
I
2
C Processor Companion with F-RAM Devices
DESCRIPTION
The FM31xx and FM3127x/FM31L27x processor companion families are similar in many ways, however
there are a few key differences. The original FM31xx family operates over a wide voltage range, 2.7V to
5.5V. The FM3127x and FM31L27x families are split into 5V and 3V versions, respectively.
The following table shows the new part numbers and the corresponding densities.
Density Original Family New 5V Fa mily New 3V Fa mily
256Kb FM31256-G FM31278-G FM31L278-G
64Kb FM3164-G FM31276-G FM31L276-G
16Kb FM3116-G FM31274-G FM31L274-G
4Kb FM3104-G FM31272-G FM31L272-G
The original and new devices are mostly the same. The interface, features, and densities are the same,
however, there are a few differences: (1) operating voltage range, (2) trickle charger, and (3) RTC crystal.
Register 0Bh, FM31xx
Reg Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Register 0Bh, FM3127x/FM31L27x
Reg Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
(1) The new FM3127x devices operate from 4.0V to 5.5V and requires only one bit to program two VTP
trip points, 3.9V and 4.4V. Similarly the new FM31L27x devices operate from 2.7V to 3.6V and also
requires only one bit to program two VTP trip points, 2.6V and 2.9V. Register 0Bh is shown above for
original and new devices. Note: This is code compatible with the original FM31xx since Bit 1 in the new
FM3127x/FM31L27x is ignored.
(2) The new devices have a higher trickle charger current for the base setting (FC=0). They also have a
new Fast Charge mode (FC=1) that is user-selectable. This Fast Charge mode allows a 0.1F supercap to
be fully charged in 15 minutes. Note the VBC bit remains in location Bit 2, Register 0Bh. The FC bit has
been added to location Bit 5, Register 0Bh. If the trickle charger is disabled (i.e. VBC=0 and battery used
as backup source), then the trickle charger changes are irrelevant.
(3) The new families use a 12.5pF crystal, a more commonl y available type than the 6pF type. Otherwise
there is no change to the RTC.
All other specifications have remained the same for the new family of companions. The new families are
software backward compatible.
The original and new devices are available in the “green”/RoHS package, designated as “-G”.
AN405 - Comparing FM31xx and FM3127x/FM31L27x
Dec. 20 07 Page 2 of 2
COMPARISON TABLE
The difference s b e tween the original and new familie s are summarized below.
Original Family New 5V Family New 3V Fa mily
FM31xx FM3127x FM31L27x Comments
Operating
Voltage Range 2.7V to 5.5V 4.0 to 5.5V 2.7V to 3.6V Split into 5V and 3V
versions.
Voltage Detect
Trip Points 2.6V, 2.9V,
3.9V, 4.4V 3.9V, 4.4V 2.6V, 2.9V New devices have only
one VTP bit in register
0Bh.
RTC Crystal
Type 6 pF 12.5 pF A more common crystal
Trickle
Charger (min -
max range)
5 – 25 µ
µµ
µA 50 – 120 µ
µµ
µA, FC=0
200 – 2500 µ
µµ
µA, FC=1 Added Fast Char ge
mode (Register 0Bh, bit
5). Increased current on
standard sett ing.
