why am i ruining 16f628 chips???

[buckeyes1997]

okay i got a new order of 4 chips and have managed to ruin 2 already. the problem is very bizzare. i have a USB programmer that works perfectly with other chips ive tried but i cannot seem to program the 628 more than once.

if i write the code to it and then plug it into the circuit it works and displays to the LCD until i remove power. then when i reapply power it works fine again. when i remove the chip and try to burn updated code to it the programmer says it cannot recognize the chip and from then on the chip doesnt do anything....even in the circuit.

what could be wrong with this? am i somehow turning a fuse on that wouldnt let me reprogram or something? it has me scratching my head..

thanks
matt

[ninjanick]

Do you have the PROTECT fuse set? Try without if you do and see if you have the same issue. Also, are you powering the device from the ICD or a separate power source? Check your MCLR pin.

[arunb]

Did you get the supply pins correct. Pin 14 goes to +5V and Pin 5 to ground.

Also ensure the supply is +5 Volts.

Also ensure you are not driving +5 Volts to pins configured as outputs.

thanks
arun

[buckeyes1997]

hey guys thanks for the suggestions.

i connected the mclr pin thru a 10k to +5v and ground it to reset the pic. i thought i turned all code protection fuses off and am fairly sure i did that correctly. i would think that the circuit would fry the chip after say 5 minutes if it was a wiring issue with the lcd. the only other thing is the programmer. i use a USB programmer to burn the chips. it is recommended to run on 16v but i have been using 12v without any problems. even at 12v i can burn the 628's the first time without any problems and i was able to burn/reburn the 16f84s all day on 12v so i dont think that is the problem.

are these chips more prone to static than the older type or something?

[Mark]

Well did you try the programmer at 16V? Also, call Microchip tech support and tell them what you are seeing. What kind of programmer is it?

[Chas]

What kind of programmer are you using? You say it is a USB programmer, but what brand and model? Also, how about showing us the fuse settings that you are using?

[buckeyes1997]

[PCM programmer]

Here's the schematic.
http://www.kitsrus.com/gif/k150_sch.gif
They're using a 7812 regulator. Here's the data sheet:
http://www.national.com/ds/LM/LM340.pdf
The dropout voltage is 2.0 volts. The KIT150 schematic shows
a silicon diode on the ground pin, to raise the output to +12.6v.
So you would need a minimum of 14.6 volts input. Do you
have that ? I'd suggest you get a 16v power supply as spec'ed
by the kit.

Edited to add:
Actually, he's also got a reverse-protection diode in series
with the 12v input pin, so you really need 15.2 volts minimum.
If you're using a 12v, 500ma wall transformer, then it probably
really puts out somewhere between 14.5 and 16.5 volts, depending
on the load. It might be enough, or it might not. You should
measure the output voltage of your power supply with a meter.
If it's not at least 15.2 volts, you need to get another power supply.

[Mark]

[buckeyes1997]

well that would explain why it works perfectly with the 16f84 but why does it consistantly work the first time with the 628's?? after the first time it doesnt even recognize the chip and it is actually damaged because even my other programmer wont read/burn it after that. i will try burning one and removing it then sitting it on my desk for 10 minutes and then try to reprogram it. that should rule out the external circuit doing any damage to it.

i will try measuring the voltage with it loaded. im not sure where ill be looking for those voltages since they wont be on the Vss/Vdd pins right? which pins should have the 16v on them and is it only while programming it??

thanks guys
matt

[PCM programmer]

You should measure the voltage at the input power connector to
the Kitrus board. Measure it right at the pins, before it goes through
any components on the board.

[asmallri]

[buckeyes1997]

you guys are all geiuses. turns out the 12v was enough to burn the chip the first time but no enough to erase it the next time. i was able to increase the voltage to around 16v and now all is well. well at least with the programmer..ha.

many thanks