[iyonix-support] Re: Some Guidance Required Please
- From: Dave Higton <davehigton@xxxxxxxxxxxxx>
- To: iyonix-support@xxxxxxxxxxxxx
- Date: Tue, 03 May 2011 20:46:30 +0100
In message <51cbb06213julie@xxxxxxxxxxxxxxxxxxxxxxx>
Julie <julie@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
> I have never been able to understand why the voltage is this critical.
> Neither does my OH. We are both electronics engineers. Back in the
> good old days, TTL would tolerate 4.75 - 5.25V, 7V absolute max.
> CMOS could run anywhere between 3V and 18V. (I know that many of the
> more recent ICs run at 3.3V.) Why is the Iyonix so sensitive?
JB has given the real answer: the Iyonix isn't particularly
sensitive, but its PSU ages and malfunctions.
To attempt to partly answer your question about logic supply
voltages: B series CMOS (now long obsolete in any real sense)
was most unusual in operating over such a wide range of supply
voltage, but you should go back to some data sheets and see
just how much its speed varied over that range. Almost all
other logic families in the last 40 years have been specified
to work with a supply voltage that had to be within +/- 5% of
nominal. Too low and it was too slow (out of timing tolerance);
too high and other problems would come into play: overheating,
reduction in lifetime, ground bounce, and generation of excessive
overshoot and undershoot. These last effects can cause circuits
to malfunction. I've seen it happen at first hand. The usual
cause is, I believe, spurious conduction paths through the
substrate. (The most powerful example I saw was a serial port
of a DSP being disabled by undershoot; when we disabled and then
re-enabled the port, it operated again until the next undershoot
event. Repeatedly enabling it without first disabling it made
no change.)
To design reliable electronic systems, it helps to have some
understanding of the components that are not explicitly present
in the schematic diagrams. PCB traces have to be considered as
transmission lines when logic signals with short rise and fall
times have to be propagated through them, for example.
Dave
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