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I'm using wiringPi for PWM on BCM18. I tried to use this to generate a STROBE-equivalent signal to turn ON the LED for the Camera Capture. My intention is to synchronize the LED-ON to shutter opening (note: LED-ON is BCM18 = 0). In order to do that, I need to re-sync the PWM reference point whenever I detect it going off-sync. For that I use pwmWrite() to redefine the new ON time. However, after several experiments I discovered that pwmWrite() does not change the duty-cycle immediately (i.e., does not restart PWM with new reference); instead it waits for the on-going cycle to be complete and then the new value is effected!! Using pwmSetClock() after pwmWrite() also did not yield the result I wanted - i.e., set the new PWM-Start reference with immediate effect on calling pwmWrite() WiringPi does not have a pwmDisable() and pwmEnable() calls that I can make use of. Anybody has tried this scheme and found any workaround ? Please please let me know of the tricks to circumvent the problem.

Thanks in advance - HC Srinivasa

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  • How do you perform shutter opening? Can't you do both actions using the same pin? Also check out this.
    – Dmitry Grigoryev
    Apr 8, 2019 at 15:30
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    Dmitry, shutter opening timing is not under software control; I can only specify the exposure duration, but when the shutter opens is controlled by the Camera. Ican only get an indirect indication of 'when the shutter closed' through encoder-callback in piCamera. So, I'm trying to synchronize LED-ON timing based on encoder-callback time.
    – Srinivasa HC
    Apr 9, 2019 at 4:50
  • Why do you need to use PWM? Are you trying to react to a GPIO status change by sending PWM? Or are you setting a GPIO and then sending PWM?
    – joan
    Apr 9, 2019 at 8:32

1 Answer 1

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I would not use PWM for this, as it creates more problems that it solves. Since you have a callback (from the shutter) which is called each time you need to generate a new LED pulse, you could generate a new pulse on demand every time, either in software or in hardware. If the duty cycle is essentially constant, you can use a pulse generator (AKA monostable multivibrator), which can be implemented with either an Arduino or with a dedicated chip like the famous 555. If the duty cycle has to be adjusted, I would just toggle the pin in software, perhaps from a separate thread.

If you wish to keep using PWM, you'll have to implement a control algorithm which estimates the delay between the LED pulse and the stutter opening window (e.g. from picture brightness) and gradually adjusts the PWM period to minimize that delay.

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  • Dmitry, I used GPIO - turn ON at start of capture and turn OFF in encoder callback. But here, I cannot use continuous capture. Because of this, the LED-ON time is coming to about 60ms for a framerate of 10. If I were to use continuous capture, then I do not know the shutter-open time to turn-ON the LED. So I use encoder-callback as a reference point that indicates shutter-close, and estimate the time at which the LED shall be ON for the next frame and start a PWM for that.
    – Srinivasa HC
    Apr 9, 2019 at 9:41
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    "estimate the time at which the LED shall be ON for the next frame and start a PWM for that" - my point is that you should generate a single pulse rather than starting a PWM. You get a callback for every frame, right?
    – Dmitry Grigoryev
    Apr 9, 2019 at 9:50
  • Yes I get a callback every frame. How do I program a GPIO to go active in future - say, after 90ms? Using a timer is more error prone because of its own jitter. That's the reason I used a PWM. You have any idea for future-setting of a GPIO?
    – Srinivasa HC
    Apr 10, 2019 at 5:59
  • Really having hardtime in restarting PWM from a new sync point. When I change duty-cycle, it gets applied only after the current PWM-cycle is complete; so, I just cannot change the reference starting point after starting it. I even tried modifying wiringPi code to do a PWM0_DISABLE and re-enable it inside pwmWrite() function. But somehow it doesn't oblige to start a renewed PWM cycle from the time I write a new value. It always waits for the current cycle to be over. I'm not getting any idea as to how to make it forget about the ongoing cycle and start a new one from NOW. !!
    – Srinivasa HC
    Apr 11, 2019 at 9:08

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