Homicidal 877 user needs help

[DavidFMarks]

Well, herewe are again thanks for further suggestions. I have now changed the xtal for a 4Mh resonator with changes to the config to match. Still as dead as a dodo (rather like the 6 877's which now return the Code programming error at 0000 and have presumably died)
RB7 (pin 40 remains at about .3 volts and for some reason pin 39 is at 4.9 I have incidentally checked and rechecked the solder connections and since the circuit initially worked perfectly there can't be much wrong with the layout. In fact with the darlington chips removed thereis nothing but the supply, the two decoupling caps (0.1) a pull up on mcr (pin 1) and the resonator and led connected to rb0. Is this annoying or what (The local cats are all going into hiding) andthe divorce can't be far off .. Ah well.

[DavidFMarks]

Just tries repeating the tests using the lab2x board which was also working when I started this project ... same result. So.. using three separate circuits - New pcb, Lab2x and breadboard with simplest of all jobs i.e. flashing one led (Both of the first two circuits have aleady worked previously !) still nothing . There has to be something incredibly stupid here .. only I think perhaps I am too stupid to see it !

[BobK]

Hi David,

Just a quick question. Have you metered your power supply for the proper output voltage? I know it sounds elementary but based on what I'm reading here nothing is working! Nothing on either of your 3 boards. Try removing the micro from the socket, apply power and read for proper voltage at pins 11 & 32 are + 5v and 12 & 31 are grounds. Don't forget to turn the power off before putting the chip back in. Possibly a electrolytic capacitor is in backwards?

Did your PCB's ever work? Maybe there's a short or shorts on the board. Check all of the pins on the 877 for shorts to +V and to ground.

I guess maybe it's Miller time! Sit back or walk away for the night and get a good night's sleep and try fresh again tomorrow. We've all been there! It's not a pretty site.

I know this is no consolation but it does help to walk away some time.

BobK

[DavidFMarks]

Thanks Bob,
Voltages all check out OK. I noticethat on oneofthetwo chips I am using the output on RB0 -RB7 is about 0>4 volts on one chip and >16 on the other ..is this reasonable ?
I am using three boardsto try thechips on. One is a melabs LabX2, couple of weeks ago I plugged this in and reading the manuals (yesreally !) realised that I could do ICP through the ten pin header direct from my epic programmer. I had a simple,single led flashing program working and could change the timing in circuit .. Great ! so far so good. Then that circuit began to behave as if there was a poor connection sometimes it didn't work an pressing the chip eould restart it, gradually it died altogether. I then built my "Christmas star" PCB which has the d and c ports connected to darlingtons to switch the - side of the leds and port D connected via darlingtons to reed relays to swotch the + sides of the leds which are in a matrix with diodes. I buit a ten pin header into this pcb connected as per the melabs board to enable me to ICP. This part of the circuit I have never got to work so i programmed the 877 direct and the circuit worked like a dream with main power from a car battery charger (to power all the leds) with a 7405 providing the 5 volts for the 877 circuit (I have since tried it with a completely independent 5 volt supply from another board). Then came the disaster ! whilst reprogramming I inserted the 877 into the pcb one pin out of alignment and the whole thing died.Replacing the 877 and the 7805 did nothing. The only sign of life I have ever had sincewas as mentionedin earlier post when touching pin 13 (Xtal) would produce a flashing led where I expected it (albeit with a somewhat variable mark space ratio). Thinking this indicated an oscillator problem I followed the othe piece of advice and inserted a 4m resonator and changed to program (i.e. osc set to xt and 4)
Once again it is absolutely dead. Sorry to bore you with all of this but the full story must some where contain the clue that points to the stupid thing I must have done ! I have had the good night sleep (and the whisky) and tomorrow am off to Brussels for a three day break with wife and friends where I shall sulk a bit more and drink to much wine and Belgian beer ! Many thanks for the trouble you have gone to to keep me sane !
cheers David

[james]

David,

You had mentioned that the configurations were a mystery to you. Why dont you post the configurations settings that you've been using. If you had it working once, then the code would appear to be ok. Next items to check are Configurations and hardware..and you seem to have gone through plenty of scenario's with the hardware.

I've had problems where a simple config setting has driven me nuts, and once the setting was changed it worked like a charm. If you have MCLR set as RESET then you must have a resistor tied to +5v. I havent seen any talk of this other than mister_e's post. Write up a list of what your currently using, I think it may be beneficial. Im learning aswell, my knowledge with pic chips is very limited. If you think this may be irrelevent, I apologize for wasting your time.

Good luck to you
James

[DavidFMarks]

Hi James,
I have always suspected the config settings but must have tried most of the permutations by now ! The settings as at the last failed test were:
oscillator XT (whilst I have the resonator in circuit)
power up timer is enabled but nothing else under the config menu
i.e.Watchdog timer,Brown out reset lvp Flash program write are all disabled
Can't see any reference to mclr but I do have 1K to +5 voltson pin 1
Thanks for the suggestions
Cheers
David

[BobK]

Hi David,

Maybe we are getting somewhere afterall. You stated that you put in a 10 pin header to allow ICSP. Did you also add the diode to block the voltage from going to the rest of your circuitry? I use ICSP to program a product I make and I do not have any +5vdc connection between the programmer and the pcb. I only use the mclr, ground, RB6, and RB7 pins. When I did another project several years ago with an 877A I also had problems doing ICSP so I gave up on the idea. I have since worked out the problems by just using the connections I gave above. I have read in several places that the PGM pin should be grounded during the programming process.

Do you have a simple breadboard that you can setup a very basic 877A with the mclr to +v thru a 1k resistor, put a led/resistor on RB0, add the 4mhz resonator and then test out the blinky program? This could help isolate whether its the PIC or your boards. You should be able to create the blinky program without adding any config settings. Just use the one that's in the MCS program.

HTH,

BobK