DAC1 pin goes HIGH during a reset/reboot

Can anyone recommend a good way to keep the DAC1 pin LOW (0v) during a reset?

Mine is high (+3.3v) during a reset/reboot. I have the Photon controlling an air pressure regulator valve, so every time I reset/reboot the Photon, the valve opens and blows compressed air into the industrial rig controlled by the valve.

I haven’t tried the digital output pins yet. Does anyone know whether they are high or low during a reset?

Thanks in advance.


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Let me ping one of my engineers who might know this answer for you as well as one of our elites that has answered questions like this in the past. @BDub or @ScruffR can either of you help on this?


I haven’t checked this yet, but we usually hear of such troubles with some of the D-pins (D3, D5 & D7) that get pull-ups temporarily attached during boot-up to provide JTAG access.
For DAC this is new to me, but I’ll check.

Thanks guys. Will await your results.

It’s really easy to replicate … just hold down the reset button and measure DAC1 pin.

Cheers, Dave

My Photon DAC1/DAC2 pins just float when RESET/DFU_MODE/RUNNING_TINKER. I can easily overcome this with a 150k ohm resistor to ground. Even after I’m actively writing to DAC1 as an OUTPUT via analogWrite(), if I press RESET with the 150k ohm attached, it will return to 0V immediately.

If your DAC1 pin is always 3.3V, and you can’t overcome that with a 1k ohm resistor to ground, then it’s shorted to 3.3V internally/externally. 1k ohm should load the DAC output to about 3V when the DAC is set to 4095.

All pins should float during reset, except for the JTAG pins as ScruffR mentions, those have weak pullup/down impedance of ~40k. You can disable them as well with a local toolchain build. (see JTAG reference mentioned). There are no JTAG pins aligning with DAC1 or DAC2 though.

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