[Arm-netbook] [EOMA68] RS232 on SoCs

[Henrik Nordström]

lör 2013-08-31 klockan 10:23 -0400 skrev Ryan Mullen:
> Hi Luke,
> 
> Sorry, just to clear up my confusion - at what point does RS232 come
> into play here?

It doesn't. Only terminology confusion. Have given up on trying to
correct people misuing RS232 term in these contexts.

>  I was under the impression that the UART lines on the
> EOMA connector were TTL, and so a USB<->serial adapter that can handle
> 0-5V signals was what was needed (and is what I am using).

The UART lines are not TTL compatible. Only 3.3V I/O supported.

>  RS232
> signalling is definitely outside this voltage range and shouldn't be
> used to connect to the EOMA interface, correct?

Correct. You will likely see some magic blue smoke escape if you attemt
to connect a fully powered RS232 using max voltage supported by RS232.

> I'm not sure I understand why connecting, say, 5V to the UART RX line
> with the A20 powered off would cause any problems (unless there is an
> excessive current situation). I typically work with lower end micros
> and I've never worried about it before...

It's not about excessive current. It's about powering the chip via I/O
pins instead of power pins. Now the Allwinner chips do not really power
up via the I/O pins, but there is at least sufficient power leakage back
to the power rails to cause problems for the AXP power management
controller chip, and maybe some internal I/O functions as well.

Same problem is seen on at least i.MX233, and AM335x, and from what I
can understand many other SoCs as well. Sometimes even clearly specified
that you must not apply any power to I/O pins before powering the chip.

Regards
Henrik