FM28V020 - 32Kx8 F-RAM
SRAM Drop-In Replacement
The FM28V020 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 V
DD
is applied. When /CE is low, the device automatically
detects address changes and a new access begins. It
also allows page mode operation at speeds up to
15MHz.
A typical application is shown in Figure 2. It shows a
pullup resistor on /CE 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 /CE pin tracks
V
DD
yet a high enough value that the current drawn
when /CE is low is not an issue.
V
DD
FM28V020
R
CE
when /WE is low is not an issue. A 10Kohm resistor
draws 330uA when /WE is low and V
DD
=3.3V.
V
DD
R
MCU/
MPU
CE
WE
OE
A(14:0)
DQ(7:0)
FM28V020
Figure 3. Use of Pullup Resistor on /WE
The FM28V020 is backward compatible with the
256Kbit FM18L08 device. Operating the FM28V020
with /CE toggling low on every address is perfectly
acceptable.
MCU/
MPU
WE
OE
A(14:0)
DQ(7:0)
PCB Layout Recommendations
A 0.1uF decoupling capacitor should be placed close
to pin 28 (V
DD
) and the ground side of the capacitor
should be connected to either a ground plane or low
impedance path back to pin 14 (V
SS
). It is best to use
a chip capacitor that has low ESR and has good high
frequency characteristics.
If the controller drives the address and chip enable
from the same timing edge, it is best to keep the
address routes short and of equal length. A simple RC
circuit may be inserted in the chip enable path to
provide some delay and timing margin for the
FM28V020’s address setup time t
AS
.
As a general rule, the layout designer may need to
add series termination resistors to controller outputs
that have fast transitions or routes that are > 15cm in
length. This is only necessary if the edge rate is less
than or equal to the round trip trace delay. Signal
overshoot and ringback may be large enough to cause
erratic device behavior. It is best to add a 50 ohm
resistor (30 – 60 ohms) near the output driver
(controller) to reduce such transmission line effects.
Figure 2. Typical Application using Pullup
Resistor on /CE
For applications that require the lowest power
consumption, the /CE signal should be active only
during memory accesses. Due to the external pullup
resistor, some supply current will be drawn while /CE
is low. When /CE is high, the device draws no more
than the maximum standby current I
SB
.
Note that if /CE is grounded, the user must be sure
/WE is not low at powerup or powerdown events. If
the chip is enabled and /WE is 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
V
DD
yet a high enough value that the current drawn
Rev. 2.1
June 2011
Page 6 of 14
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
