Re: [Paparazzi-devel] module period accuracy ???

[Chris]

Hi Christophe
I will try the voltage measurement because LIsa might have many serial 
ports but
the tx pins that are needed are used for usb i think.
The timer interupt method looks promishing so i need to check chdk 
performance with a pulse generator at work first just to be sure.
CHDK has a resolution of 10ms so pulses of 50ms for taking picture,
100ms for zoom in and 150ms for zoom out should be easy to distiguish.
CHDK detects when the usb power is on and then when the power (5v)
is removed it reports back the pulse width
I will also recheck the code because if we are talking about 20-30ms off 
maximum
then it should work fine.
You know the difference between theory and practice :-)

"Theory is where you know everything but nothing works.
Practice is where everything works but nobody knows why.
In my lab we combined both Theory and Practice, nothing works and nobody 
knows why."

Meanwhile i use the pulse count mode which works fine and
after some tweaking is very reliable and not unacceptably slow after all.
Of course i use a 3.3v to 5v level converter to drive chdk from the LED 
output for safety.
Chris



On 10/17/2012 10:12 AM, address@hidden wrote:
> If you have spare serial ports (lisa has pleanty) you could use a very low
> bitrate serial port (I don't know how low it can be: 300 bits/sec
> 10bits/sec?)
>
> Or you could have a very small function be called in an interrupt routine /
> timer.
>
> But I think something else is the real problem and your module could do
> just fine even if the timing is sometimes slightly (usually not more than
> 1/500 of second) off.
>
> How fast does the CHDK loop run versus your module loop? How do you know
> CHDK is not sampling exacly while the LED changes since the 2 loops are not
> time synced? In order to be observable, the *CHDK loop needs to be at least
> twice as fast as the module*, and you need to sample each bit twice to not
> miss a bit an even more like three times if you also need to know its peak
> level. -->
> http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem