[Arm-netbook] [EOMA68] RS232 on SoCs

luke.leighton luke.leighton at gmail.com
Sat Sep 7 03:12:26 BST 2013

On Fri, Sep 6, 2013 at 10:02 PM, Henrik Nordström
<henrik at henriknordstrom.net> wrote:
> 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.

 that looks distinctly.... odd!  like it's going to entirely block
transmit.  but then i realised if Tx goes to 0V that current flows ...

 ok do you mean 3.3V with a resistor?  so the connection goes:

 3.3V - resistor (e.g. 1k) - AW-Rx & also off to diode >| - UART Tx

like that?  that would almost make sense, except that Tx had better
pull down an extra 0.7v below the logic level expected by the Rx pin
otherwise not a lot's going to happen.

is that actually the case?  do 3.3V UART cables pull down to 0V (or
close to it)?


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