ICSP Circuit Question
Russell McMahon email (remove spam text)
> > I am trying ICSP ... I found this page ....
> > www.sprut.de/electronic/pic/icsp/icsp.htm
> > .... just wondering about the 200k
> > resistor ?
> The 200Kohm resistor is sufficient to keep MCLR high during normal
> operation, but still allow the reset switch to short the line to ground.
> Personally I would have used a lower value like 10K to reduce the chance
> noise pickup, especially since this line goes to an external connector.
In fact 200k is too high for guaranteed operation - it violates a little
This is a special spec developed by Microchip just so we can say that even
Olin is wrong sometimes. (I know he's been wrong before but I can't think
when :-) ). (As opposed to me who is often enough wrong all by myself
without Microchip having to gang up on me). This largely only applies if the
circuit is used as shown to subsequently run the processor (which is usually
what is implied by ICSP)
It goes like this. The /MCLR (aka /RESET) pin has a specified leakage
current of +/- 5uA worst case. This means that it CAN theoretically sink up
to 5 uA through the pullup. The /MCLR pin has a 0.8Vdd spec for a logical
high. At 5 volts you are therefore allowed to drop no more than 0.2Vdd
across this resistor = 1 volt. For this R = 1v/5uA = 200k. BUT we here have
a series diode which takes part of the available 1v. It will be less than
the typical 0.6v due to the small currents involved. At 0.5v diode drop say
that leaves 0.5v for the resistor or 100k maximum.
As Olin says (showing that even when he's wrong he's still right!) a value
nearer 10k would be more comfortable in practice (assuming that other
factors not discussed here don't come into play.) It's also worth noting
that in the real world the 200k will probably usually work even though it
violates specifications. Just don't blame uChip if it doesn't.
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