Teensy 3.6 unwanted DAC output

[EyeofHorus]

I am amplifying the output signal from DAC pin A21 with an op amp to get to the 0-5V range instead of 0-3.3V. During startup of the Teensy I can measure a voltage after the op amp that increases up to about 4.8 V, which I really don't want to have. If I measure directly on the A21 pin, on the other hand, the voltage is 0 and also disappears from the op amp output.

This behavior seems to be independent of the specific code I upload to the Teensy. If I don't use the pin in the code at all, or if I declare it but write no value to it, the output voltage stays at 4.8V and doesn't disappear after the setup is done.
Is there anything I can do to make this go away? Preferably some piece of code that would make sure pin A21 is actively at output 0 as soon as the device has any power.

 

[PaulStoffregen]

First try adding a resistor between the signal and ground. 10K or 22K might be a good starting point.

About configuring the pin right at startup, look for "startup_early_hook" in mk20dx128.c. This code runs as soon as the chip boots, while it's still running from the internal RC oscillator (neither crystal is oscillating yet) and pretty much nothing has been initialized.

 

[EyeofHorus]

Awesome, thank you. Adding the resistor solved it. I'll still look into the startup_early_hook you mentioned, that sounds very useful for other things.