CMOS Battery-backed RAM Update

Microcode (Control) Limited

Having read Paul Anderson’s excellent article: ‘16K CMOS Memory Extension for the Nascom 2 Main Board’, I would just like to mention a few points of interest.

Experience gained in manufacturing the 32k CMOS Battery-backed Board (see our advert. – will the Editor forgive us this ‘plug’?) has taught us the following lessons with regard to the standby current of these amazing memory chips (HM6116—).

1. The standby current is extremely temperature dependant and for a particular memory chip this may vary over a ratio of 1000:1; we consider an average of 1 micro-amp per degree centigrade over the zone 15 to 25 degrees C as about typical for the standard ‘P’ series.
2. The best standby current is obtained by pulling the address, read, write and chip select signals up to V standby while in the standby mode. This obliquely hinted at in the specification sheets for these devices. This is most conveniently done using 10K DIL resistor networks. Without these pull-ups the ‘in-circuit’ standby current will often reach over 150 micro-amps.
3. In order that the pull-up scheme works, it is wise to use tristate buffers on these lines. However, this introduces yet another current sink, namely the leakage of the output transistor pairs in the tristate drives.
   We use 74LS126 quad tristate non-inverting buffers with active-high enable. Beware of the standard 74126 TTL part, as this has a wicked diode between the output pair and the Vcc to catch the unwary (me!). Also the S.G.S version of the 74LS126 seems to be leakier than other manufacturers.
4. If you want really long data retention periods the standby currents of the ‘LP’ versions are of at least an order of magnitude better.
   The ‘LP’ versions are functionally identical but have passed a special ‘low standby current’ test during the final stage of manufacture. The slightly higher price is well worth the cost in terms of this performance in our opinion.
5. Just recently the Toshiba equivalent device (TC5517AP) has dropped in price to compete with the Hitachi chip. The Toshiba chip does seem to be far superior in all parameters and our own tests would seem to verify the manufacturers claim.