Cheap protection...

[Mike (Perth, Western Australia)]

At 08:59 AM 26/3/2000 +0000, you wrote:
> Use a voltage divider and do the de-bounce/filtering in software.

Yeah agree with that for the most part unless the signal range is
limited - then use a cheap transistor like BC547 etc.
ie Atennuate first then amplify to the 5v range required.

> A simple signal diode from input to Vcc will protect your uP as the Vcc
is low 
>imperance.

<snip ascii art>

I can see a problem with this. Although one hopes the 5v rail is low
impedance - remember its connecte to your other logic so its not good
practice to use this as a sink for any incoming noise - it will be
reflected across the voltage to your CPU and eprom etc - it might be
OK for a range of frequencies but, 5v regulators are
notorious for parasitic oscillations, leave the o/p bypass cap off and
the device will go into a 65MHz resonant mode - looks awful !
This can also happen if external noise is injected onto the 5v rail,
which is what you want to avoid.

Its also possible to trigger the 5v bypass transistor into saturation
if the spike on its 5v output is too high as this temporarily reverse
biases the bypass transistor first then causing a 'over-correction' which
can lead to momentary high voltage - it all depends on how bad the noise is.

In my design I use a high current 5v regulator (3 or 5Amp) with a reverse
diode  (UF4004) across output back to input *and* and big fat transzorb
across the 5vrail near any inputs with a choke as well on i/p to regulator
and on the common pullup rail near any inputs...

This is all in addition to using high impedance input rs-232 receivers
to translate the dirty automotive stuff down to the nice 5v (or 3v)
levels needed for stability...

Oh and usually prefer to do some minimal filtering with a low esr input
cap due to c=vdi/dt, ie Cap doesn't allow instantaneous voltage changes.
The capacitance of even 100nF has far better transient response then
the capacitance found in a 1n4148.

Don't get me wrong, I'm sure your approach will work for most of the time,
but any high voltages from ignition impressed on the line could make
your 5v flakey for a time - long enough for a failure or partial failure
that looks like s/w errors...




Rgds


Mike Massen

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