Triggering ADC

Nimblest

I'm using the Quarto as a PID bias controller for an optical modulator. To get an error signal for the PID loop, I need to sample a photodiode with the Quarto's ADC at a specific (triggered) time. This ADC rate is nominally 25kSamp/sec, but needs to be synchronized with another signal source that is the master clock for the system. Is there a way to do this?

The only thing I can think of is to use an interrupt trigger routing to enable the ADC for one sample and then immediately disable the ADC until the next trigger event. I'm not sure that will be fast enough, so I'm reaching out to see if there's a better way to do this.

Ben

There are a couple ways to do this, I'm afraid nothing very elegant yet.

Have the ADC sample data at some relatively fast interval and store the measurement in a global variable. When you want to sample the ADC, send the Trigger 1 input high and configure the Quarto to fire an interrupt when Trigger 1 goes high. In this interrupt function you can read the global variable that is storing the latest ADC value. If you want to sample at 25kHz (40us) and you configure the ADC to sample at, say 250kHz (4us) then there will be to 4us of jitter on the time between the ADC measurement and the trigger input. Maybe this timing is good enough.
Use the Quarto as the master clock. This is (for now) undocumented but a future release will add this with a cleaner helper function, but if you run
```
analogWrite(12,128);
analogWriteFrequency(12,freq); // freq is a float in Hz
```
Trigger 1 will output a clock at the frequency specified by the float freq. The lowest frequency is 15 Hz and the frequency will be rounded to the nearest frequency that is an integer division of 66 MHz. Hopefully you can then use that clock to sync other hardware. You can use the ADC interrupt to trigger the start if the output clock frequency is at a different from the ADC collection rate.
We are working on an update that will enable the Quarto be accept a 10 MHz clock into the Trigger 1 port and sync the internal Quarto clock to that reference so the ADC collection and the MCU processing would be in sync with this external reference. If you would like to beta-test this new functionality when it is ready, please let me know.
Taking an external clock signal and using it to control ADC acquisition is a useful feature that I think the hardware would support. We will look into seeing if we can add this functionality in a future update.

I hope this helps and sorry I don't have a cleaner solution at this time.

Nimblest

Ben Thanks for the triggering ideas.

"Yes", I'd like to beta test an update that allows a 10 MHz clock input. Keep me posted.

Cheers,

Ben

Sorry this took longer than we hoped. But there is a beta firmware update that supports syncing the Quarto clock to a 10 MHz clock input on trigger 1. If you use our beta url for the Arduino software (see https://qnimble.com/Quarto/Quick_Start for details), then the new 1.6.1-beta1 release has a new example under clocks called "UseExternalClock" that is a quick demonstration of using the Quarto with an external clock.

This new feature requires an update to our new beta firmware. Anyway who want to try it out, please contact support directly (link) for the latest beta firmware and instructions on how to update your Quarto.

Nimblest

All good. Thanks for working out a beta solution. I'd like to try it out. I'll contact support for the latest beta firmware.