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'[PIC] The ICSP puzzle'
2007\04\15@173446 by PicDude

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I have a PIC-based project installed in my car and it has the ICSP wires
routed out of it and wired to a DB-9 connector.  But now, some months later,
I can't find the DB-9 pinout.  Would it be possible (using a multimeter) to
identify which wires correspond to which ICSP pins, without tearing back into
the car?

Some specifics on the application circuit:
- PIC16F913
- MCLR is not connected to anything (using internal MCLR).
- Vdd is at 5volts.
- RB7/PGD is not connected to anything.
- RB6/PGC is connected to one side of a tact switch and the other side goes to
ground.  Yes, I have access to the tact switch.
- Vss is connected to a ground wire coming out of the gauge.

With this, I figure I should be able to determine Vss, Vdd and PGC relatively
easily.  From the code, RB7/PGD is currently setup as an input, and MCLR
defaults to an input.  I'm thinking I can connect a 0V-5V signal to any input
pin though a resistor and being an input, the voltage at the pin should
follow the external signal.  But with both MCLR and PGD being inputs, would
it be possible to differentiate between these two?

Cheers,
-Neil.

2007\04\16@100620 by Gerhard Fiedler

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PicDude wrote:

> Some specifics on the application circuit:
> - PIC16F913
> - MCLR is not connected to anything (using internal MCLR).
> - Vdd is at 5volts.
> - RB7/PGD is not connected to anything.
> - RB6/PGC is connected to one side of a tact switch and the other side goes to
> ground.  Yes, I have access to the tact switch.
> - Vss is connected to a ground wire coming out of the gauge.
>
> With this, I figure I should be able to determine Vss, Vdd and PGC relatively
> easily.  From the code, RB7/PGD is currently setup as an input, and MCLR
> defaults to an input.  I'm thinking I can connect a 0V-5V signal to any input
> pin though a resistor and being an input, the voltage at the pin should
> follow the external signal.  But with both MCLR and PGD being inputs, would
> it be possible to differentiate between these two?

Are both of them open inputs? No pull-down/pull-up? (Shouldn't there be
one?)

If both are really open inputs, you probably can take the U/I curve above
5V (using a suitable series resistor) and should be able to see a
difference (without destroying anything): the diode to Vdd is missing on
the /MCLR pin.

Gerhard

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