CMOS Battery-backed RAM Update
Micracede (Control) Limited
Having read Paul Anderson’s excellent article: ‘16K CMOS
Memory Extension for the Nascom 2 Main Board’, I would just ike
to mention a few points of interest.
Experience gained in manufacturing the 32k_—CHOS
Battery-backed Board (see aur advert. – will the Editor forgive
us thiz ‘plug’?) has taught us the following lessons with regard
to the standby current of these amazing @enory chips.
1. The standby current is extremely temperature dependant and
for a particular memory chip this may vary aver a ratio of
1000713 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 atin the
specification “sheets for these devices. This 15 most
Conveniently done using 10K DIL resistor networks. Without
these pull-ups the ‘in-cireuit’ standby current. will often
reach over 150 micro-ampe.
3S. In order that the pull-up scheme works, it is wise to use
trastate buffers on these lines. However, this introduces:
yet another current eink, namely the leakage of the output
transistor pairs in the tristate drives.
We use 7415126 quad tristate non-inverting buffers with
active-high enable. Reware of the standard 74126 TTL part,
as this has a wicked diode between the cutput pair and. the
Vee to catch the unwary (me!). Also the 5.6.5 version of the
7ALS126 seems to be leakier than other manufacturers.
4, If you want really long data retention periods the standby
currents af the ‘Le’ versions are of at least an order of
The “LP versions are functionally identical but have paused
a special ‘low standby current’ test during the final stage
Of manufacture. The slightly higher price ts well worth the
cost in terms Of this performance in our opinion.
5. Just recently the Toshibe equivalent device (TCS5I7AP) has
dropped in price to comete 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