Cheap protection...

[Mike (Perth, Western Australia)]

At 09:36 AM 27/3/2000 +0800, Bernd Felsche <bernie at perth.dialix.com.au> wrote:
>Scotty Grover writes:
>>'Scuse me for butting in, but a CD4049 (Inverting) or CD4050
>>(Non-inverting) may work better than a 74C14.  The CD's (Powered by
>>5V) accept 15V at their inputs.

>Thanks.
>
>You're excused for butting in. :-)

The only 'slight' problem with that is if the Vcc rail to the device is
lower then the input spike (or whatever) then it forward biases the onchip
parasitic diode - depending on the type of CMOS you are using it could
cause a latchup anywhere on the chip...

If however the input resistor values are high enough then the resulting
current is lower, less likelihood of latchup. Also best to add a cap from
cmos device signal input to ground for these reasons:-

a.	From c=Vdi/dt there is far less chance of a transient passing a
	current into the CMOS parasitic diode.
b.	Provides some minimal debounce, saving some CPU overhead.
	From t=1.1RC you can obtain quite nice debounce values without
	s/w overhead.
	Eg. 1Meg and 1uF gives a time constant of about 1 second
c.	A nice blue cap breaks up the clour scheme somehwat ;-)

>That brings the parts count down nicely; just a series resistor
>required ahead of the buffer for minimal protection.

Hopefully the one extra cap won't upset the parts count too much, I think
a 1uF 35V electro is around 20c or so in small qtys.

If you wanted to be real sure you didn't expose the inputs to any big spikes,
I'd add a 6v8 transzorb in parallel with cap  and a 10K or so resistor from
this conenction to the CMOS input. The transzorb's have a nice strong cutoff
and act as a short at anything above 6.8v - they are quite commong and
cheap in qty...

Also IMHO, if you want the most cpu time avalable for nice-eties such as
PID and kalman filtering (I'm talking EFI here) then its worth bending
the rule about "having s/w do as much as possible". Although a s/w debounce
is trivial - if you have a lot of them it can interfere with best task
scheduling to some degree. With basic input filtering for discrete inputs
provided by resistors and caps the CPU has *full time* to do the interesting
stuff for compensations and a consistent 'time' term to scale the PID
functions appropriately etc ie. Without time spent on debounce, the time
term in a calc is synchronous and less chance of instability...




Rgds


Mike Massen

Ancient Sufi saying:
        "Should your God save you from adversity, choose another God"

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