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Thread: One shot timer

  1. #1

    One shot timer

    When you answer, please specify which Teensy versions your answer can apply to.

    How to immediately schedule an output pin to flip in X uS from now. X can be from 400 - 2800uS(microseconds).
    sub 1uS resolution preferred, but 4uS resolution of micros() would be acceptable.

    The general case might use delay, or constantly check elapsedMicros(), but I am interested in scheduling an interrupt to occur or use peripherals because I cannot guarantee acceptable loop times.

    This project has further constraints.
    The scheduling task above must be non blocking and happen quickly within the ISR of a pin change falling interrupt.
    Other things that must happen within that same ISR:
    Immediately set output pin High,
    compute X = constrain(time since previousRising + offset[+/- 400 - 2800uS])
    Offset is a frequently changing value based on data received via serial.
    I would like to avoid turning off interrupts at any point if at all possible.
    Each input pin has a corresponding output pin. Input Capture seems like the right way to go but Im not clear on how to use it, and ...
    This same process is happening on a minimum of 4 other pins at the same time.
    The better solution maximizes the number of pins I can do this on.
    It is highly likely that some input pins will trigger simultaneously but have different offset values, thus different output timing.
    This sketch uses PulsePosition library to read input PPM stream at the same time, so the scheduling approach must be compatible.
    Edit: I forgot to mention, this one shot timer/interrupt scheduling task comes up about every 10mS on each pin and they are semi regular. Their clocks do creep.
    Last edited by slomobile; 08-15-2019 at 08:41 PM.

  2. #2
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  3. #3
    Quote Originally Posted by luni View Post
    Looks promising but both libs use ftm0 and conflict. I've filed an issue against PulsePosition a while ago suggesting ability to select which ftm is used. Any guidance there, or generally how to write libraries which select alt peripheral numbers? PulsePosition rarely requires more than 1 input and 1 output making the other FTMs the better default choice. Since PulsePosition requires mods for Teensy 4 anyway, perhaps that feature could be worked in as well?

    Trying the example DelayChain and adding #include <PulsePosition.h> result in

    In function `ftm0_isr':
    /arduino-1.8.9/hardware/teensy/avr/libraries/PulsePosition/PulsePosition.cpp:267: multiple definition of `ftm0_isr'
    /tmp/arduino_build_145602/libraries/TeensyDelay/TeensyDelay.cpp.o:/arduino-1.8.9/libraries/TeensyDelay/src/TeensyDelay.cpp:74: first defined here
    /arduino-1.8.9/hardware/tools/arm/bin/../lib/gcc/arm-none-eabi/5.4.1/../../../../arm-none-eabi/bin/ld: Disabling relaxation: it will not work with multiple definitions
    collect2: error: ld returned 1 exit status
    Last edited by slomobile; Today at 12:10 AM.

  4. #4
    Senior Member+ defragster's Avatar
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    Looked at this again - TeensyDelay looks like it could be handy. Will be nice working with T_4.

    As far as the TIMER used - I scanned the github and it indicates this :: If you want to change the timer used by TeensyDelay please edit line 22 in the file config.h according to the description given in the file.

  5. #5
    I saw that note. Unfortunately I need every available channel for TeensyDelay and only 2 channels for PulsePosition. At least I can try to use TeensyDelay as a model to modify a local copy of PulsePosition.

    Edit:Nevermind. I thought FTM0 was the only one with 8 channels, but FTM3 does as well. It compiles now.

    Code:
    // D: Default, X: available
    //
    //      TIMER           | Channels|        Timer is available for           |
    //                      |         | T-LC | T3.0 |  T3.1 | T3.2 | T3.5 | T3.6|
    //----------------------|---------|------|------|-------|------|------|-----|
    #define TIMER_FTM0 1  //|   8     |      |  D   |   D   |  D   |  D   |  D  |
    #define TIMER_FTM1 2  //|   2     |      |  X   |   X   |  X   |  X   |  X  |
    #define TIMER_FTM2 3  //|   2     |      |      |   X   |  X   |  X   |  X  |
    #define TIMER_FTM3 4  //|   8     |      |      |       |      |  X   |  X  |
    #define TIMER_TPM0 5  //|   6     |      |      |       |      |      |     |
    #define TIMER_TPM1 6  //|   2     |      |      |       |      |      |  X  |
    #define TIMER_TPM2 7  //|   2     |      |      |       |      |      |  X  |
    #define TIMER_DEFAULT -1
    
    // If you need a special timer, please replace "TIMER_DEFAULT" by a timer from the list above
    #define USE_TIMER TIMER_FTM3
    Last edited by slomobile; Today at 12:47 AM.

  6. #6
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    Glad that it works.

    sub 1uS resolution preferred, but 4uS resolution of micros() would be acceptable.
    Just to make sure: Did you see the possibility to manually set the prescaler in line 23 of the config? Setting this to small values gives you a sub Ás resolution. However, a smaller resolution will also reduce the maximum delay time (16 bit timer)

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