Hi Jim,
Thanks for the questions.
- "How well synchronised do the different Dues have to be"
We'd like to get it below 20us. However, there are some obstacles to this. Firstly, total capacitance, line driver & logic gate delays will add some delays. We have not yet calculated those, but they are - presently - thought to be within our limits. Our second problem is that - for a new design - we'd like to go radio (probably some variant of 802.11 for its broad availability). That puts the cat among the pigeons because now that very fast response time is becoming increasingly difficult to achieve. Again, we haven't done the maths, but we are pretty sure that there are going to be complications. Our middle name... LOL.
- "How did you get the 1 Mhz figure"?
Well, 1,000,000 interrupts for 10 Khz leaves 100 interrupts for each cycle. In other words, 10 Khz @ between 0% and 100% duty cycle in 1% increments.
However, as these units will - initially - only see service inside our studios, we are fine with 5 Khz (which corresponds to exactly 5 pulses of light for a 1/1000 shutter speed on our cameras) - something which, thanks to the Due's fast clock - we have already achieved.
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Unfortunately we only have a small hobby scope at our disposal and its resolution is inadequate for testing the Sync. However, there is a solution (we think) in that we are considering using an 8 input AND gate which will allow us to test the synchronicity of up to 8 Due's by the following methodologies:
- Assume pulses are initially out-of-phase by - say - 50%. That gives a net duty cycle of 25% on a 50% PWM (the AND gate is shut for the rest of the time because not all inputs are high).
- Issue a SYNC pulse to the Due's
- Verify that the AND gate output pulses now show a 50% duty cycle.
Whilst the above is inadequate to verify whether or not we were able to keep our response time sub 20 us, it will show us that our SYNC system is working, and that all the PWM signals are now lining up nicely.
The reason "jitter" isn't that much of an issue for us is, that at lower shutter speeds (and @ 10 Khz), each exposure is likely to be subjected to - say - 80 light pulses (1/125 sec). Even if we missed an entire pulse (not overly desirable, but not catastrophic either), we'd only experience a variance of just over one percent; not distinguishable by the naked eye.
Hope that answers some of your questions (as well as some you haven't asked, but are directly related).
Rgds, J.