[Arm-netbook] [EOMA68] RS232 on SoCs

[Philip Hands]

joem <joem at martindale-electric.co.uk> writes:

> 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

I have one of these:

  http://www.ebay.co.uk/itm/USB-TO-RS232-TTL-PL2303-CONVERTER-FOR-RASPBERRY-Pi-ADAPTER-CONSOLE-CABLE-/121070687261?pt=UK_Computing_Other_Computing_Networking&hash=item1c3060141d

it worked well on the first A10 board I had, when connected via a
breadboard.  Now that I've been using it with the MEB it only
occasionally works, perhaps dependent on the order ////i plug things in,
but more probably at random.

I'll reassemble the breadboard setup when I get back to the UK, and see
if it's something about the MEB or whether the converter is playing up.

I've tried just plugging GND and RX (from the converter) in, which I'd
think would provide no power, and still I generally see no output from
the fel-loaded uboot -- that's despite the uboot then going on to boot
Debian from SD to the point that I can log into it over the USB-OTG's
network.

So, on current evidence I cannot recommend it, but I'll be able to tell
more when I try it with just the 3 wires plugged into the PCMCIA
connector via my breadboard.  It would probably be interesting to
provide power on the MEB's 5v input to see if that works better, which
I've not yet got round to either.

Cheers, Phil.
-- 
|)|  Philip Hands [+44 (0)20 8530 9560]    http://www.hands.com/
|-|  HANDS.COM Ltd.                    http://www.uk.debian.org/
|(|  10 Onslow Gardens, South Woodford, London  E18 1NE  ENGLAND
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