[Arm-netbook] [EOMA68] RS232 on SoCs

[joem]

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)?

---
This entire thread sounds wrong.

First off, it is impossible to bring down a 3.3V CPU with external 3.3V
devices connected and IO disabled. All CPUs start up in this way, have
to initialise and then turn on the IO ports. Never crashed any ARM product yet.

Second, it is impossible to connect SoC TX pin to a 3.3V UART and
see the board fail. I do it all day every day with no problem.

The A20 will go from 0V to 3.3 and they are normally
active pull down and pull up for the speed - which means they can both pull down 0.3V
of the ground or 3V of the 3.3V rail.
If you are having to fit pull ups, or finding devices crashing
when connecting UART, its probably a 5V USB UART device. They
can be marketed as capable of 3.3V operation. Measure the UART TX output voltage
with nothing connected. It should be under 3V. If it goes above 3.3V, then
it will crash the CPU as its a 5V UART. The UART RX voltage should be below 0.5V
with nothing connected. If its above that, it could be a 5V device. If its above 3.3V then it
definitely is a 5V device.