Okay guys, I have a "fix" to this.  I'll explain what I went about doing...

FAE visited me and spent several hours with me trying to fix this.  Not=20
sure what happened when I scoped it earlier (images I sent), but now we=20
got a decently clean sine wave (when sleeping) and a bit of=20
noise/shakiness when the display was on.  PS lines showed some noise=20
too.  This time the equipment was moved to a desk with no other=20
electronics, so I must've had something else being picked up=20
previously.  Noise-tested a similar circuit using a 16F1936 and the=20
noise was a *bit* lower when awake.  Definitely learned a lot about=20
crystals and loading capacitances from him.

Anyway, we tried (a) earthing the crystal case; (b) using a cylindrical=20
32.768khz crystal instead of this SMD type (both of which I have used=20
quite a bit before); (c) jumpering ground traces to stiffen the power=20
lines, (d) changing/adding bypass caps and electrolytics on the power=20
rails; (e) using a TO-220 7805 (as this is the first time I've used=20
these TO-89 78L05CPK's); (f) using a TO-92 78L05, (g) adding series=20
resistors of 140k and 280k; and none of these worked.

I did also try not updating TMR1H in the ISR to get 0.5-sec pulses (for=20
the flashing colon) and instead just read TMR1H in the main code loop=20
regularly to determine if the flashing colon should be on or off any=20
time.  Still no dice.

What worked is when I removed the TMR1H read in the mainloop. =20
Apparently access this is causing some issue, and I've gone back to the=20
datasheet and errata and still have not found anything that says this=20
could be a problem.  Yes I do have TMR1 set to 8-bit read/writes.  I'm=20
going to have to live with a static colon for now, but 1.5 days later=20
it's still clocking accurately.  I'll emailed the FAE so let's see what=20
he finds out about it and I'll report back.  Thanks for all the help.

Cheers,
-Neil.


On 6/8/2012 6:21 PM, IVP wrote:
>> Cases not earthed -- was not aware that that could/should be
>> done
> It has fixed occassional mysteries. Personally cured problems
> caused by, such as, the position of wiring to an LED display,
> and the proximity of a relay
>
>> should have a sine wave, rather than square.  See attached
> eeeuuuuwwww. That's overdriven. Did you say this was the first
> time you'd used these particular crystals with the 18F ?
>
> Suggests 100k - 200k series resistor
>
> http://www.microchip.com/forums/m313266.aspx
>
> Atmel, but would be relevant to PICs, see Section 4.2
>
> http://www.atmel.com/Images/doc8333.pdf
>
>> In sleep mode, I get the same partial-sine waveform.  If this really
>> is the issue now, then why is the wake-mode waveform changing?
> Possibly the 32kHz crystal is broadcasting noise into the die or the
> primary clock source
>
> Joe

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.