This is wierd

[christoph]

Hi all,
I am somewhat baffled but I wondered if someone could throw any light on the following - this seemed to be the only heading I could raise this topic on:
1) A few months ago I had a working Flowcode UART signal at 9600 baud using a breadboarded PIC16F88 (also works with PIC16(L)F88) proven by my logic analyser showing the correct hex and binary codes being output on a relevent button press. 2) This was then PCB'd for use at a later date, tested OK and put to one side for future use. 3) When I tried using it a week ago it did not output the UART signal. The logic analyser registered the 0 to 3v step when the power was switched on and just stayed at 3v (analyser set to 'live scroll'). 4) I took the chip off the PCB, stuck it into the original breadboard set up which I hadn't used since - it didnt work. Tried another chip re-programmed off my original flowcode (which runs fine in simulation and transfers to chip with full verification etc etc) - zilch! Programmed another chip not previously used zilch. 4) Another 'new' chip programmed successfully and was put onto a new breadboard, connections were made to PLUS, MINUS FOR 3V from a stabilised PSU and the voltage double checked with meter probes to the relevent legs AT THE CHIP BODY, 1k and subsequently tried 6k and 10k resistor from pin 4 (MCLR) to PLUS and a 3volt input signal onto A0 PIN 18 (representing in my Flowcode a switch) to give the UART output as 34 in decimal etc out of pin 11. ZILCH! At this point I realised the pin 1 end of the chip was very hot and as nothing was working even with this basically simple set up which had been ok a few months ago. So I bought new chips, found my Flowcode LED flasher , programmed a new chip all well in simulation and verified download to chip ok, put it on yet another good breadboard, connected plus and minus and MCLR and LED via appropriate, pre-tested LED and again - nothing but chip getting very hot in the same way. Nothing registering on the analyser from the output I selected on flowchart or any i/o at all after trying several flowcode changes. I have left out the hours, details, checking and searching on the web within Flowcode and Microchip and microchips in general and Matrix 'if your project is not working' etc and even got some more chips. I cannot fathom out why every one of these chips is overheating so badly either - my original working breadboarded circuit worked first time and there was no DISCERNABLE heat at all. With the successfull programming of brand new chips and the minimal connections frequired for blinking an LED, I am totally stumped HAS ANYONE ANY IDEAS PLEASE?? The only other thing I tried was to take PGM pin to earth but it made no difference when I added that with a new chip - still produced nothing and got very hot. Surely, even if I just put + and - on this chip it shouldnt fry with or without a programme?

Any ideas gratefully received

Many thanks

Chris

[kersing]

Chris,

The obvious questions:
- did you check the voltage from your power supply?
- are you using a DC supply?
- are you sure you haven't swapped ground and positive supply power?
- is the power supplied clean? I.e. no high frequency signals with out of bounds signals super imposed on the signal?

Best regards,

Jac

[petesmart]

Also it may be helpful to post a closeup photo of your rig... It may give some clues

[christoph]

Hi fellars,
Yes - to all that and scoped the 3.0 volts power supply and connection to chip at pins (right way round and right at point where they enter the chip body) for anything untoward and lots of non-obvious things like using 2 other breadboards, checking the continuity of each wire and connection etc etc. However, this PSU runs all my other Flowcoded things quite happily. The thing I really dont understand at the moment is why the chips are overheating and this might be the reason the chip is not operating but it is SO unusual for chips to be faulty and anyway I have just bought replacements and they are doing the same thing. I have even tried 2 LR44 silver oxide button cells which together give 3.1 volts nominal and still have same problem. I'm not sure how to post a photo on here but let's just take the situation for blinking an LED with a Flowcode program for PIC16(L)F88 (the program simulates and works correctly - just checked on a completely different chip to this '88 ), silver oxide batteries giving 3.1 volts DC and connected the right way round, each connection checked for continuity, LED checked operational and connected the right way round etc., SO the only breadboard connections are +ve and -ve, MCLR pin 4 via resistor (checked as 6.25k on my meter) to +ve, 3mm red LED from A0 pin 17 via 680 ohm resistor (that lights up ok when connected across the 3.1v batteries) to +ve and thats it!
Can I pose a direct question: If I connect a brand NEW and non-programmed PIC16(L)F88 chip to plus and minus from 3.1 volts battery supply as above with NO OTHER CONNECTIONS, should it overheat?? - I wouldnt have thought so personally but I dont know for certainty...

[Benj]

If I connect a brand NEW and non-programmed PIC16(L)F88 chip to plus and minus from 3.1 volts battery supply as above with NO OTHER CONNECTIONS, should it overheat?? - I wouldnt have thought so personally but I dont know for certainty...

No this should certainly not be happening. Chips getting hot is a bad sign. If that's all your connecting then it sounds to me like a duff chip. Where are you buying the chips from?

My guess is that there is a short or other related problem that is causing the overheat problem. Do you have capacitors across your voltage supply rails to ensure there is no noise or nasty oscillation getting to the micro? Of course if your powering direct from a battery then you shouldn't really need the caps, the battery acts as the cap but it's probably worth having at least a 100nF cap in situe.

[christoph]

Hi Benj,

Thanks for your reply to my last question - I didn't think there was a heat problem if just supply connected but 'clutching at straws' at the moment. Will post when sorted for the benefit of anyone coming across this unusual situation.
I have had a cap(s) connected across the supply line and as you say from those button cells I wouldn't expect they were really needed - have also taken them off each time just as double jepordy but they are not usually a problem of course and needless to say, it didnt make any difference. These '88's have all come from RS so reputeable supplier but I nevertheless, can only conclude that there is an internal problem with the chip. I have used one of three I just ordered so will try connecting a second new one just to the battery 3.1v dc and if that one heats up then I will contact RS and send back the third one for them to investigate as they were all of the same batch.

Best regards,

Chris

[Benj]

Hello Chris,

Just had a look in the 16F88 datasheet and the F version of the chip supposedly won't run at 3.1V. The LF chip should be fine though.

An under voltaged chip could theoretically be trying to do something and repeatedly browning out or in a funny state which may potentially cause heat up issues though not very likely especially on a blank chip.

The shaded area shows the workable voltage to clock speed on the F and LF versions of the chip.

16F88Voltages.jpg (42.88 KiB) Viewed 9930 times

[christoph]

Hi Benj,

Yes it is the 16(L)F88 low power I am using and maybe thats why if battery wa a bit down the F88 did not appear to be working on my original project after a few weeks.
HOWEVER, I hadn't had time to check chip with supply only connected earlier and just got back in but could not resist a quick check before 'turning in' and unbelievably NO HEATING at all!! Tried the 'dodgy' one again and it heated up as before so IS obviously a dud. So unusual that I had not really considered the problem was that but in the abscence of any rational explanation and as a last resort I was going to connect the supply only but I thought I would check first. Your answer that it shouldnt heat up made it worth a try and it didn't so many thanks indeed for coming to my rescue. Programming etc will have to wait until tomorrow but I have temporarily included the LED flasher in my Flowcoding for the project for immediate indication of basic chip operation being ok when a dedicated input to chip is made.

So many thanks contributors, much appreciated,

Best regards, Chris