/dev/ttyTHS1 TX working unreliably

I’m trying to send and receive data to and from a microcontroller connected to the Jetson Nano’s TX and RX pins. RX seems to work fine, but TX is very unreliable. The connected device will receive incorrect data most of the time (e.g. getting 208 or 130 instead of 1). I tried decreasing the baud rate from 115200 to 9600, which helped somewhat, but the device is still receiving incorrect data a large portion of the time. I’ve disabled nvgetty and ModemManager. When I use a USB to Serial adapter, the problem disappears completely.

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