Alternatives for battery-backed RAM

Published:2011/7/28 21:54:00 Author:Phyllis From:SeekIC

Many microprocessor boards include a static RAM that stores a variety of system settings. It is often desirable to keep these settings when the power supply is turned off or interrupted. For this reason a backup battery is mounted on these boards, which provides standby power to the SRAM.

At first sight, this seems a simple solution, but it is far from ideal. Batteries take up a fair amount of space on the PCB. They also need to be checked regularly for any possible leakage and that they supply the correct voltage. On modern processor boards a flash memory or EEPROM is often used, neither of which require a backup battery. Older boards can be given a new lease of life by replacing the SRAM and batteries with a modern alternative such as FRAM.

Ramtron International Corporation has introduced two ICs, the FM1608 and FM1808, which are ideal for removing the need for backup batteries in older systems, and which are of course very useful in new designs too. These are a special type of nonvolatile memory. Ferroelectric RAM, which is being developed further by the manufacturer. A detailed description of the technology can be found on their website at www.ramtron.com/aboutfram. The manufacturer guarantees at least 10 billion read/write cycles and a 10-year data retention for these ICs. The best characteristic of these memories is that they are pin compatible with standard 8 K x 8 (FM1608) and 32 K x 8 (FM1808) SRAMs and EEPROMs, and can therefore be used in existing designs without having to make any drastic modifications.

There is just one aspect of the design that has to be taken into account when a ’normal’ SRAM is replaced by an FRAM: the CE-signal of an FRAM is used to clock an internal address latch, as shown in the timing diagram. This means that the IC requires a falling edge on this pin after a valid address has been put on the address bus, otherwise the address won’t be recognized!

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