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
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.
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.
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.
If you want really long data retention periods the standby
currents of the ‘LP’ versions are of at least an order of
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.
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