ESP32 behaving differently when UART connection is closed

[Dommar92]

I'm using an ESP32 devkitc V4 (azdelivery) with <Arduino.h> for reading six piezos for a drum sample pad and detecting peaks to send MIDI signals.
I use the second HardwareSerial on TX Pin 17 and everything works fine when it's powered over USB and a UART terminal (platformIO) runs on default Serial for logging (currently it logs nothing, it runs just to make it run).

This is when the fun begins:
When I close the terminal connection the ESP starts to behave strange:
Only half of the pads are sampled and the performance is awful.
It seems like it is slowing down the processor and/or shutting down half of the ADCs.

My wiring:

Code: Select all

Pad 1:  Pin 4    (ADC2_CH0) - not working
Pad 2:  Pin 0    (ADC2_CH1) - not working
Pad 3:  Pin 2    (ADC2_CH2) - not working
Pad 4:  Pin 15  (ADC2_CH3)  - very bad detection
Pad 5:  Pin 13  (ADC2_CH4)  - okayish
Pad 6:  Pin 12  (ADC2_CH5)  - works good, but not perfect

HWSerial 2 TX: Pin 17 (U2TXD)

Mode Button: Pin 23 (works well)
Mode LEDs:   Pins 19, 18, 5 (works well)

I use a (by me) modified version of leandcesar/peakdetection to prevent overhead when using six detections simultaneously, that also works well.

I may post some code, but I guess this is a mainly hardware based error, I did not find any problem like this.

Thanks for reading