Calibration: off by 1 error

[mungewell]

As mentioned on #4 we have a way to assess the XTAL accuracy by performing a calibration process, and logging the results.

I have/am writing scripts to automate this process and previously noted that a unit could settle on calibration values which differ by 1 unit - which represents 1/256 fractional division of the CPU clock.

I previously though that this was a set-up issue (ie only at start), but now I have a couple of logs where the calibration value changes during the process to 'jump' between the two values. This suggests that there is something wrong with the way the code monitors/tracks the input (from UltraSync)...

and

Note: Multiple units monitoring the same input signal, so I don't think that this is the UltraSync glitching.

[mungewell]

Also note that '1 unit' represents a change of around 2PPM, this is a tiny inaccuracy.

[mungewell]

Pushed changes to the 'pcb-rev1' tree, which I think solves this issue.

[mungewell]

Haven't see this occur in recent testing of Rev2.0,closing.