review of timing details

I set up a couple more hardware flags on my software so I could see things like, edge detections vs rollover detections vs PC transmissions, etc. It all looks perfect to me. I took maybe a dozen different samples, looking for problems, and everything lines up well. Edges are detected where they should be, high’s are detected properly, transmissions happen every 8 bits, etc.

I also manually counted cycles for both the ISR and the main routine, and made sure that I wasn’t exceeding the number of cycles in the ISR. I also checked to see if the ISR is starving main, it’s not — main is soo tiny, it only takes at most 200-300 ns to process the “send” which is really all main does.

I did find one problem. I noticed that the number of stored bits BEFORE I reached the PC send routine was reaching exactly 9 bits(never 10 or greater) for some reason. I also checked to see if an edge or a HIGH was consistently triggering causing the shift register to overflow. I found that an edge-trigger is consistently overflowing it, and that a HIGH never caused it to overflow.

This means I’m losing one bit on a regular basis — the lost bit just gets shifted off the left hand side. I made a correction by forcing a PC xfer IN THE ISR right before the 9th bit gets added. Normally main handles the PC comm. This did fix the problem I was seeing, but it really did not improve the results at all. I was hoping that would be the “big problem”. I want to narrow down WHY that was happening though, and see if that sheds light. It has something to do with the timing of an edge vs the transmission.

I guess the problem I’m having now is that everything looks kosher. I want to put up some more timing images showing just how well it looks like its working.

Onward and forward.

About the author


Amateur Electronics Design Engineer and Hacker

One Comment

  • Can you sort of post a sketch or something like scan of a handdrawn block algorithm of your code? This sometimes helps me – you know, drawing all these circles, blocks and arrows.