On 11/05/13 09:10, Lachlan Gunn wrote:
From https://github.com/akafugu/vetinari_clock :
The firmware has a 'random' sequence of pulses which over 32 seconds
moves the second hand 32 times. This sequence is long enough to give
the appearance of randomness without unnecessarily consuming
processor cycles. I chose 32 seconds as it means the pattern is
constantly
offset from the 60 second rotation of the minutes. The 'random'
pattern
is 128 steps (and is therefore checked 4 times a second). This means
the
clock moves between 0-4 times within each actual second. By moving
more or less in each of the 32 seconds the clock looks random,
however
the sequence always moves the clock exactly 32 seconds in the 128
steps.
Not quite what I guessed, but similar enough.
So there is no long-term drift, it seems, beyond that of the crystal. I've
pulled the pseudo-random tick sequence out of the source code and found the
Allan variance, shown here:
http://imgur.com/wF9F8pI
Not completely a surprise for a phase-modulation, as phase-modulation
have that slope in the ADEV.
As a systematic modulation, MTIE analysis would be of more help, as you
have a looped sequence and analyze the pattern you would see how the
maximum time error will be a flat line beyond 32 s, making it easy to
believe that on long-term there is no increase in time.
You could also attempt the TDEV, which will just give a straight line
with wrinkles just as the ADEV.
Cheers,
Magnus
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