Cheap protection...

[Orin]

> 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.

This is a good idea - a simple transistor inverter.
I've used it to monitor the coil drive signal.

> 
> > 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

Another problem is that the logic input probably has a protection
diode to Vcc too.  Which will conduct first?  Then some devices
(MC145051 AtoD in particular) don't like their protection diodes
being turned on.

I agree totally with not shunting to the 5V line.  I used a diode
to a shunt regulator set to about 4.7V in one design to keep an
MC145051 happy in over-voltage conditions.

For a simple on/off though, a series resistor and a 4.7V zener diode
works for me.  Add a capacitor in parallel with the zener for
filtering if necessary.

What series resistor?  Well an OEM ECU I have around uses 10k.

Orin.
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