On Tue, 22 Oct 2002, Dwayne Reid wrote:

*>At 03:49 PM 10/22/02 -0500, llile@SALTONUSA.COM wrote:
*>
*>>4. What if the input voltage drove an emitter follower?  Emitter of PNP to
*>>the input voltage, base to 5Vdc, and collector to MCLR?  Hmm, this is
*>>getting better.  If I could draw ASCII circuits I would draw this, but I'm
*>>bad at it.
*>
*>The approach I have used in past is very similar to what you mentioned
*>above: cheap PNP transistor (2n4403) with C to GND, B to Vdd, E to
*>MCLR.  MCLR goes 1 diode drop above Vdd which has not been a problem (so
*>far).  Note that this will NOT raise Vdd like a diode to Vdd might.
*>
*>But I've used zener diodes before and they have worked OK as well.  I don't
*>even remember who makes the ones that we use - they come from Future or
*>Arrow and have nice a sharp knee at less than 1 mA.
*>
*>You can also use a 2 diode 2 transistor clamp similar to the one that Peter
*>Pere has mentioned previously - this will constrain MCLR between Vdd & gnd.

Actually only one half of it is needed (the upper half), but as you said,
for MCLR it's ok to go slightly beyond Vdd so your approach is better,
using only one transistor.

My full clamp circuit is useful when there are AD inputs in use on the
chip. The half clamp is used for video sync separation ...

Peter

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