Wouter van Ooijen wrote:
>>  For
>> readback, the target chip PGD is connected thru another 2Kohm
>> resistor to
>> the comparator input of the controller PIC.
>
> Which value do you put on the other input of the comparator? I hope
> you do switch the programmer-PIC output that drives the PGD line to
> input?

The other comparator input is derived from 1/2 the Vdd level with a little
filtering.  This allows the threshold to adjust as the Vdd level is
adjusted.

I don't remember whether I let the PGD driver float or not.  That is another
reason for the 2Kohm resistor.  Both the controller and target chip can be
driving PGD at the same time with no ill effects.  The target chip drivers
can easily overcome 2Kohms to ground.  That's only 2.8mA with Vdd at the
maximum of 5.5V.

In any case, the PGD readback signal at the target chip looks nice and
clean.


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