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Thread: Which pin is low/gnd on reset ?

  1. #1
    Member educa's Avatar
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    Which pin is low/gnd on reset ?

    Hi there,

    I want to use a teensy 3.6 to connect to a laser diode with ttl input.

    This laser is at this moment connected to A9 (the ttl enable) and I can fire the laser by setting this pin HIGH and stop firing by setting it LOW.

    But..... upon reset, it looks like this pin gets HIGH (or undefined) or about a second. This means than when I reset the teensy, my laser gets a GO signal and fires.

    Is there a way to safely solve this? Can I somehow make sure that upon reset this pin is pulled to gnd? Maybe on the teensy itself or maybe using external stuff like a pull down resistor? (if a resistor, then what value would be a safe value) ?

    I don't neccesarily have to use pin A9 to shoot the laser. If another pin would be better suitable then thats OK too , but upon reset I would have to be sure that the laser doesn't see a TTL high signal on this pin and doesn't shoot.


    Kind regards,
    Bart

  2. #2
    Senior Member
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    What is happening is during reset the Teensy pins are disabled with no current flow - which is good from the power consumption perspective but means your laser input is free floating. It looks like they wired it internally to pull up to make it 'fail safe' from the tech support perspective (if people don't connect the TTL it still works) and 'fail dangerous' for anything else. Simple hack is to add a reasonably aggressive pull down resistor to it (1k, if that doesn't work try closer to 400ohm). Better would be looking at the laser diode driver and see if there is a simple pull up you can remove to make the device fail safe.

  3. #3
    Member educa's Avatar
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    The diode driver is totally potted so that won't allow me to do it. It is indeed intentionally made that way that people who don't need ttl will have the laser always on without having to connect the ttl signal to vcc

    SO... a 1k resistor connected between pin A9 and gnd won't kill the teensy ?

  4. #4
    Senior Member
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    Teensy pins are good for 10-20 ma. 3.3V/0.01A = 330 ohm, but also discounts any current through the module hence suggestion of 1K.

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