With regard to data retention, the following points should
Data corruption is more frequent while Vcc is rising (during
power-up) than while Vcc is dropping (during power-down),
however it can happen during either transient phase. Corruption
usually takes the form of one or two bits in a few (apparently
random) locations being set high.
The causes are many and various but, as Paul Anderson
points out, are predominantly due to the unpredictable behaviour
of the Bus control signals during the transient phase of
power-up and power-down. Corruption is not always easily
detected, we ourselves use a check-summing routine after having
initialised memory contents with the pattern AAhex 55hex.
Those readers who have purchased our memory boards may have
wondered why on receipt your board is full of AA 55. If you
really want to be smart you can run this program at 2000hex by
selecting IC 11-12 to block 2 (if you did not overwrite it in
your enthusiasm to use the board).
To overcome this danger of corruption, we developed a fast
switch circuit to disable read and write lines whenever Vcc
falls below 4.6 volts. (see diagram) The circuit is based about
a programmable zener (IC 1) set to 4.6 volts. The reason we use
this particular device in preference to a normal zener is due to
the fact that its dynamic slope impedance is only 3 ohms. as a
result, as soon as Vcc rises above the programmed zener voltage
(4.6v) the zener draws current through TR1’s base, switching TR1
on and TR2 off.
TR2 is switched on whenever Vcc is above 0.6 volts and below
4.6 volts and while switched on pulls the commoned enables of
the tristate buffers low, thus disabling all read and writes.
The circuit switches in under 2 micro seconds.
I hope this information will have been of some interest and,
perhaps, use to your readers who intend utilising these
delightful chips. Non-volatility is an asset that, once gained,
forces one to think how on earth one survived without it.
Lastly, as a concession to Micro Power readers, we will
offer the HM6116LP-3 (150nS access time) at £4.50 and the
Toshiba TC5517AP (150nS access time) at £4.95 up until January
1983 to readers who enclose the cut-off section from our advert.
in Micro Power.