Hi people!

Last week a similar problem was discussed here in the list, but i'm having another situation, on the same topics.

Attached, is a small picture of my ICSP circuit. I isolated the main Vdd, because my programmer doesnt drain enough power to supply the entire
board.

As you can see, i didnt use the typical RC circuit that Microchip describes in their Appnote. I did it this way because my main Vdd is isolated,
and the Vdd from the programmer, is going straight for the PIC Vdd power pin, not being used to also power the MCLR'. MCLR' is connected
straight to the programmer, isolated with a diode from the main power supply.

With the PIC on the programmer, it works. With the PIC on the board using ICSP, nothing happens.
You may be wondering why i did put a 100nF capacitor between those two points???  that was the only way to put the ICSP to work.
Tested several times and it worked great. Until i took it back again.

I continued to try without the capacitor, but nothing was happening, until i read the PIC with ICSP the programmer and it would just power down it
self.... strange....

After several experiments, i noticed that when the Vpp voltage was being applied, the PIC was pulling down the Vpp line.....   i tried the
programmer alone with some wires (making short circuit between Vpp and GND while read/write) and it was happening the same.

Interesting off all, is that apart from not being able to read/write the chip, if i apply power to my circuit, it works perfectly!! perfectly..... but no
read/write access.....rrrr...

Can someone explain to me, why is this happening to the PIC???  why cant i access it to read/write???  i already fried two.

To all of you, thank you very much, and i hope i was clear enough.
Sena


*********************************************************
Antonio Sergio Sena     CT2GPW
asena@bigfoot.com

BEng Electrical & Electronic Engineering
Heriot-Watt University - Edinburgh

Homepage: www.qsl.net/ct2gpw
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