FM21LD16 - 128Kx16 FRAM
Software Write Protect Timing
SRAM Drop-In Replacement
For applications that require the lowest power
consumption, the /CE signal should be active only
during memory accesses. The FM21LD16 draws
supply current while /CE is low, even if addresses and
control signals are static. While /CE is high, the
device draws no more than the maximum standby
current ISB.
The FM21LD16 has been designed to be a drop-in
replacement for standard asynchronous SRAMs. The
device does not require /CE to toggle for each new
address. /CE may remain low indefinitely. While /CE
is low, the device automatically detects address
changes and a new access is begun. This functionality
allows /CE to be grounded as you might with an
SRAM. It also allows page mode operation at speeds
up to 40MHz. Note that if /CE is tied to ground,
the user must be sure /WE is not low at powerup
or powerdown events. If /CE and /WE are both
low during power cycles, data corruption will
occur. Figure 3 shows a pullup resistor on /WE
which will keep the pin high during power cycles
assuming the MCU/MPU pin tri-states during the
reset condition. The pullup resistor value should
be chosen to ensure the /WE pin tracks VDD yet a
high enough value that the current drawn when
/WE is low is not an issue. A 10Kohm resistor
draws 330uA when /WE is low and VDD=3.3V.
The FM21LD16 is backward compatible with the
1Mbit FM20L08 and 256Kbit FM18L08 devices.
That is, operating the FM21LD16 with /CE toggling
low on every address is perfectly acceptable. In terms
of package and pinout, the FM21LD16 is upward
compatible with the FM22LD16 (4Mb).
The /UB and /LB byte select pins are active for both
read and write cycles. They may be used to allow the
device to be wired as a 256Kx8 memory. The upper
and lower data bytes can be tied together and
controlled with the byte selects. Individual byte
enables or the next higher address line A(17) may be
available from the system processor.
FM21LD16
VDD
R
CE
MCU/
MPU
WE
OE
A(16:0)
DQ
Figure 3. Use of Pullup Resistor on /WE
NOTE: If /CE is tied to ground, the user gives up
the ability to perform the software write-protect
sequence.
Figure 4. FM21LD16 Wired as 256Kx8
Rev. 1.0
Dec. 2009
Page 7 of 14
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
