xpyttl wrote:
"Ken Scharf" wrote in message
...


diode from MCLR to VSS (Anode to VSS) and that brings the 'zero'
voltage down to -.37v or so.


You may want to check the datasheet for the specific part you are going to
use. the 16F628 datasheet, for example, specifies the absolute minimum
voltage on MCLR at -0.3v. Ditto with the 877 and 84A.

..


I added a diode in series with the VPP output from the emitter or Q1
(with the anode toward Q1's emitter) and a second shut diode
from the MCLR pin to VSS (anode toward VSS). Now the VPP voltage
at MCLR swings from -0.05v to +12.6 volts. I hope that's enough
VPP. I may have to change out the 8.2v zener, I measured the voltage
across it and it appears on the low side. I could also just connect
a 1n4001 or two in series with it to raise the vpp supply.

Anyway seems the programmer is safe now. Still need to put a scope
to the PGC and PGD lines and check they are in spec.