[Arm-netbook] [EOMA68] RS232 on SoCs

[Henrik Nordström]

tis 2013-09-03 klockan 08:21 +0000 skrev joem:

> Use RS232-USB converters that require only 3 pins to operate; and which
> operate down to 3.3V for its input.

Does not help in this case. I can reproduce AXP chip hang on most of my
boards with using only verified (and high quality) 3.3V UART cable if RX
pin is connected before applying power to the board.  Some boards are
less sensitive than others but not 100%.

reproduced on A10, A10s, A13, A20.

Only the boards with a diode and internal pullup on the RX pin surives
100%.

                    3.3V pullup to board 3.3v power rail (NOT UART cable) 
                      |  
UART CABLE TX   --|<--+-- AW SoC RX pin
                diode

This very effectively stops any current from the UART cable TX pin to enter the CPU.

Using a series resistor on te RX pin likely also help some, but not
convinced it's a good solution. It's a very low current involved.

Regards
Henrik