[Arm-netbook] [EOMA68] RS232 on SoCs

[joem]

On Tue, 2013-09-03 at 09:10 +0200, Vladimir Pantelic wrote:
> On 08/31/2013 04:59 PM, Henrik Nordström wrote:
> 
> >
> > 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.
> 
> and a 1k series resistor on the UART line is not enough to stop this?

No - as soon as you exceed 3.3V, the IO line's protection diodes start
conducting and you will end up powering the CPU through the power line
from the UART and the chip can no longer function properly.

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

http://www.gplsquared.com/eoma_boot/eoma_boot.html - photo of what looks
like a Silab CP2102 chip based dirt cheap RS232-USB converter

Works very well. And that particular model has LED to tell you if the
board is powered up.

They used to be common - but I don't see them as often
http://www.aliexpress.com/item/Free-shipping-2pcs-New-build-in-CP2102-USB-To-TTL-RS232-COM-Converter-Module-6-pin/581643413.html

The latest seems to be PL2303 chip based designs - but I can't vouch for
those ones, but someone else who uses it might be able to:
http://www.aliexpress.com/item/USB-To-RS232-TTL-PL2303HX-Auto-Converter-Module-Converter-Adapter-5V-3-3V-Output/691986109.html