Homicidal 877 user needs help

[G-R-C]

Did you happen to do anything differnt with the MCLR? Config fuses?
Another thought, if you suspect the Crystal OSC, try a resonator.
They have the caps built-in. Much simplier (in m opinion anyway).

I believe someone already mentioned it but it your switching from the
877 to the 877A this will cause you a problem if you don't change
configuration.

[DavidFMarks]

Thanks for the responses folks. I have implemented the suggestions and the plot as they say, has now thickened ! At first using my original board with the suggested changes and at first ... nothing. Then I inadvertently touched to capacito connected to the pin 8 side of the xtal my led on rb 7 began to flash , the mark space ratio was not 1 to 1 however, the on period being significantly longer. If I touch it with metal the led remains on continuously ! The caps are 22pf all the connections seem OK.
Yes I did try a resonator once before but didn't have a 20 meg (only 4) can I run the 877 20 at 4 if I change the define command. It might be worth a try as there is obviously an oscillator problem of some sort. I also inadvertently tried a 877a and as you say I got nowhere ..in fact the epic wouldn't program it. The chips I am using are new 877 20's
I wish I could use the dear old 16C84 for this job that never gave me all this grief! I am beginning to detest the 877 which is a pity since it is at the heart of a midi decoder I am using in a street organ I am building. Irony is, in that far more complex circuit it works a treat... that's what is so annoying . I shall have to go and have a stiff whisky to calm me down......

[BobK]

Hi David,

Yes you can use a 4mhz resonator or 4 mhz crystal with 22pf caps. Just change the config settings from HS to XT (actually it doesn't matter for 4 mhz but play it safe) and you should be running. The fact you mentioned just touching a capacitor made it work again makes me think you have a solder joint problem. Try re-flowing the solder to see if that clears it up. The differences between the 877 and 877A are that there are A to D's and comparators on the 877A that need to be disabled in order to use the PortA and PortE pins as digital. Check the data sheet to get the exact register names to disable.

Also make sure you have .1uf caps on both sides of the chip as close as possible to the power supply pins. These are more important on the "A" version of the chip.

I've used both the 877, 877A, and the drop in upgrade 18F452 and have to be honest, I didn't have any problems. I made 2 layer and 4 layer boards for them. I did have a wandering wire once though, and it took out my RTC chip and the uP. Also discovered the RTC crystal had to be replaced.

It's all part of the game. But it's even worse when you're on a deadline to get the damn thing working.

We're here to help David.

HTH,

BobK

[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