Measuring audio phase shift through a circuit with a PIC

[HankMcSpank]

So on the picture and data on post#15 things were looking honky-dory (you were measuring at least down to 161 counts on CMP2). Your new posts state that now CMP2 will not go below 336. Basic question; what changed?

Apologies...I've only just seen your question from a day or so ago - firstly that's very sharp of you to pick up on that detail

What changed?

I wish I knew....when it looked hunky dory...I didn't probe to deeply (I guess the - at the time - relief overcame me & I was a bit premature saying all was well!), I sort of put it to one side...a couple of nights later I came back to it & couldn't get the darned count down.

I've not changed the circuit....and have only window dressed the code (I can't genuinely say what I might have benn window dressed - but it will have been minimal!), but early indications here since getting home is that it might be related to the amount of text I'm squirting out with the HSEROUT command (which I queried myself the other night...and acetronics spotted earlier today!)

I only had a slim window of opprtunity before domestic commitments kicked in....but when 1000Hz (a nice round number to work with) was fed into comp1 I saw the count drop from 5,000 to something like 4600 when I put an Hserout in amongst the count storing! So is it related to amount of debug text I wonder?

I'm using Paul's excellent idea of keeping it simple & plagiarising the simple code he kindly provided (less his one typo! ) ...first step is to get a good solid Comp 1 derived timer count for 1000Hz (which @20Mhz should be 5000 - and indeed it is using paul's code )...I'll then augment from there (ie bring comp2 into play)