[Arm-netbook] [EOMA68] RS232 on SoCs

[Philip Hands]

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

> On Tue, 2013-09-03 at 11:19 +0100, Philip Hands wrote:
>> 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.
>
>
> Just repeat what I say earlier in case it applies.
> Avoid connecting micro USB serial cable at any time
> and bin that cable if you can (for now).

Yeah, I read that, but given that I know that Luke's been using the
USB-OTG as the main way he's testing stuff, along with the fact that I
currently don't have any alternatives, the advice doesn't help me
greatly at present I'm afraid.

Also, are you saying that the only alternative is to power it via a 5v
supply to the MEB?  (that bit was not totally clear to me)

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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