FM22LD16 datasheet(7/14 Pages) RAMTRON

7 / 14 page

FM22LD16 - 256Kx16 FRAM

Rev. 2.0

Dec. 2009

Page 7 of 14

Software Write Protect Timing

SRAM Drop-In Replacement

The FM22LD16 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.

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.

For applications that require the lowest power

consumption, the /CE signal should be active only

during memory accesses.

The FM22LD16 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 FM22LD16 is backward compatible with the

1Mbit FM20L08 and 256Kbit FM18L08 devices.

That is, operating the FM22LD16 with /CE toggling

low on every address is perfectly acceptable.

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 512Kx8 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(18) may be

available from the system processor.

Figure 4. FM22LD16 Wired as 512Kx8

A(17:0)

FM22LD16

VDD

MCU/

MPU

FM22LD16 Datasheet(PDF) 7 Page - Ramtron International Corporation

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