DCO Calibration issue

After fixing the issues with calibration related to the compiler bug, I hoped everything would go smoothly. Unfortunately that was not the case.

After calibration, when running through frequencies, I would sometime see a kind of discontinuity, where the wave amplitude would drop to zero and then build back up to the correct height again over two periods. 

I looked at the DAC charging voltage and could see a kind of asymptotic behaviour where the charge voltage suddenly rose much faster than it should, and when it reached max, dropping back to zero but still rising with the same speed. Clearly the DAC output overflowed.

DAC Voltage error. The first little drop is only due to missing calibration. The asymptotic looking one is where the interpolation overflows and voltage drops to 0.

It took some testing to figure out what was going on: For a single sample in the calibration, the dac voltage was detected significantly lower than it should, and more importantly, lower than the previous sample. This meant that the slope multiplier should have been negative. But the slope multiplier is unsigned, so it overflowed and ended up with a large positive number instead.

Then, when interpolating between the two values, we would add a large number instead of subtracting, leading to the spike (and DAC overflow) we saw.

But why did we get the wrong value during calibration?

It happens during the first cycle of a single note calibration. For some reason, we detect that the wave amplitude is too high - higher than the comparator voltage - when in reality it is not. This leads to the binary search continuing in the wrong half of the voltage range and the calibrated voltage ends up being around 32768 instead of what it should have been.

DAC voltage calibration - 7th cycle starts off too low and never reaches back up to the correct voltage.

Debugging the issue is a pain in the ass, because we are dependent of the wave being reset at exactly the same point every time. If something changes, the voltage may not cross the comparator voltage, and while it seems we have fixed the error, it still might be there.

I strongly suspect that what happens is that the voltage reaches the comparator voltage AFTER we have changed the dac and timer to a new frequency, but before the interrupt handler resets the wave. My fix was to move clearing of the comparator flag into the interrupt routine, so we are guaranteed that a comparator interrupt stems from the current cycle, NOT anything before it. It looks like it works but who knows...

I also saw another strange thing - some of the cycles are of different lengths than they should be, by as much as +2us. But then, often the next cycle or the one after that is a bit shorter. I have not been able to properly explain what is going on. I suspected that the comparator interrupt may have been part of it, that it triggering would make the program enter the interrupt routine before the timer interrupt, and then staying there for long enough to delay the timer interrupt, but I have not been able to confirm this. My fix was to disable comparator interrupts once we get it once during the calibration period, to prevent the code from reentering the interrupt routine due to the comparator interrupt being high. When this was NOT done, we would probably enter the interrupt routine repeatedly until the timer interrupt fired and disabled the comparator.

Now everything seems to work properly and I've started working on a verify routine, one that runs through frequencies where I suspect the charge voltage may be too high (such as in the middle of interpolation) and checking if we hit the comparator voltage during normal operation.

Low range sweep after calibration

High range sweep after calibration

I do however suspect that that is not the case. Had the timer been exponential between two key points and the voltage linear we would see an error, but both are linearly interpolated meaning the voltage should match the frequency fairly well. 

We have another error too - a strange falloff at the end. The charging voltage is too low even if the DAC has not maxed out. The consequence is rather low as this is in the > 20kHz range but I still want to figure out what is going on.