[Paparazzi-devel] ppm encoder technical details

[chris]

Hi.
As i continue to work on the ppm encoder i discovered that the small 
jitter present
(~3 microseconds max)
is caused by the internal RC oscillator which is of course voltage 
dependent and it's
frequency vary from cycle to cycle.
It is not coming from the input measuring code as i was thinking.
While i don't think that this is important as the performance is more 
than adequate for our use
If someone wants the jitter to fall to 0 (yes absolute zero) he will 
have to use a crystal oscillator at 16 Mhz.
I also rewrote the code and now the firmware uses assembly for the 
critical measuring code
and a filter for removing any noise from the servo measurement.
I am flying with a ppm encoder at 16 Mhz like the one i described above
and even the digital servos are dead quiet.
In my oscilloscope the pulses do not have any jitter and the whole ppm 
output is very precise.
The newer code also removes most of the jitter of the rc oscillator and 
it gives
a performance very very close to the performance of the crystal version.
If someone wants to try the new code just drop me an email.
Chris