HW: need pull-down on console uart RX line

Created by: jadonk

@embest-tech said:

We found the board goes to uboot mode when the board start without serial cables, the reason is that U46 2# pin input is floating , even if we had a pull-up resistor(R173) in the side of cpu, it will not be effective. BeagleBone AI Rev A1 increases the length of the UART1_RX signal by moving the position of the serial port, this line is very sensitive without pullup and pulldown resistor , When we touch the edge of the PCB, it brings in a lot of interference signals, causing the serial port to trigger into UBOOT mode. we suggest add a pull-down resistor on the UART1_RX signal, is it feasible to modify it in the same way as Beaglebone Black?We have tested pull-down resistor to J92 2# , this problem has not been reproduced. We suggest add a small pull-down resistor between J92 1# and J92 2# in this PP production.

In assembly of rev A1a, R173 is removed and a pull-down is added on J92 pin 2.

For rev A2, the PCB will be modified to avoid the need for this unusual assembly step.