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Thread: F876 programming questions
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face BY : Roman Black email (remove spam text)



Mike Mansheim wrote:

> When I do this "in-circuit", when I activate Vcc, MCLR will also rise to
> Vcc.  Then I am raising MCLR from Vcc instead of from ground.  Also, since
> the chip in installed on a target board, the OSC pins are hooked up, so I
> would think the chip would actually "run" for the period of time between
> activating Vcc and raising MCLR.  This is the mode of operation I'm trying
> to get a handle on.
> Thanks again for the help.


According to the Microchip ICSP app note, this is a
big no-no.

The PIC should be held in reset (MCLR low) while
it is powered up, then MCLR should go from 0v to
13v as quickly as possible to avoid the possibility
that a proper clock cycle will occur and the
program counter is incrememented. That would cause
the new program to be written at the wrong address,
ie start+x (x is how many clock cycles occured).
Microchip are quite specific about this, and they
also state that capacitances on circuits attached
to MCLR should be kept very low. I use no caps
on MCLR, instead I tie it to 5v with a 68k resistor.
-Roman

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