I don't think that it is a power up / reset problem.  The fact that the=20
heart beat runs on power up indicates that the processor is running=20
code.  It is only the UART that doesn't run.  Yet with a different=20
processor, they both run on power up.  This sounds to me like a register=20
/ variable is not getting initialized until after it is being used.  On=20
most processors it powers up in a state that allows the UART to run, but=20
on some, it powers up in a state that doesn't, as suggested by Djula. =20
You could confirm this by grounding the 100nF cap on power up, then=20
releasing it.  If it still fails to respond to the UART, but does if you=20
momentarily touch it afterward (as you indicated it does) then it is=20
almost certainly not a power up reset problem.  Take a quick look at the=20
UART initialization code and see if anything jumps out at you.

Kerry



David Duffy (AVD) wrote:
> ivp wrote:
>  =20
>>> I guess the only other thing I can try before replacing the PIC is
>>> to add some debugging pin code, maybe to the UART ISR for
>>> starters
>>>    =20
>>>      =20
>> How about disconnecting the receiver until the PIC has stabilised,
>> or something like that. Might help identify where the problem really
>> is
>>  =20
>>    =20
> I'll give that a go tomorrow. The receiver output is full of random=20
> noise when idle, but it is the same on all boards.
>
> As for power, there's not much to the circuit. A 12V battery via a=20
> 1N4007, into a 78L05 with a 100uF on the input.
>
> What's really odd about this is that replacing the chip fixed the=20
> problem on the first faulty unit. Apart from those two units, all the=20
> others work well.
>
> I'd bet money that replacing the PIC will solve the problem, but I'd=20
> really like to know why before I do that. Since it is a TQFP package,=20
> swapping them back and forth not an option.
>
> I do have brown-out detect enabled IIRC. The only other idea I have is=20
> that the power-up timer is bad on some chips. Seems unlikely though.
> David...
>
>  =20



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