[Arm-netbook] Mele A1000 serial console pinout

[Henrik Nordström]

mån 2012-05-21 klockan 12:19 +0200 skrev Peter Steenbergen:

> Hmm, it was me who added that remark into the text. I took me hours
> fiddling around why I diod not saw my Mele A1000 booting. Went to IRC,
> and they tipped me about switching Txd Rxd. It is such a smart idea
> that it could be one of my ideas ;)

No matter how you label Tx / Rx it's always a mess. There is plenty of
USB UART boards which have Tx / Rx switched, presumably with the
intention to "help" people connect them proper... and equally there is
many who label the board wrong for similar reasons.

Correct wiring is:

UART TxD -> Board RxD
UART RxD <- Board TxD
  GND    --    GND

assuming the UART is labeled properly with the functions of the UART and
not what it should connect to on the other side.

It's easy to measure which pin is which on the UART by using a
multimeter between a pin and GND. Then set a low baud rate and sending
break signal with your terminal program. If the multimeter reacts and
shows 0 volt when you send the break and 3.3V otherwise then you have
identified the TxD pin.

When the TxD pin is identified you can use this to identify the RxD pin
if there is any doubt. Simply connect the TxD pin to RxD and you should
see what you type in the terminal program.

Similarly you can measure the voltage of your UART by simply measuring
between Tx and GND. (or Tx+Rx together if unsure which is which). Note
that measuring Rx<->GND is not valid and may show any voltage from logic
leakage.

Often it's harmless to connect Rx/Tx wrongly, but only if your UART is
short-circuit protected. If both sides is without short-circuit
protection then you risk burning things.

Recommended procedure if unsure which pin is which is to

1. Identify your UART pins as per above.

2. Connect GND to the target board.

3. Connect your UART RxD pin to what you think is TxD of the target
board and perform an action that should produce output, i.e. boot the
board.

4. If no output then repeat with your next candidate for board TxD pin

5. When you see output then connect your UART TxD pin to what you think
is the RxD pin of the board. Chances are very high you get this right
now.

> Anyway, switching solved it. Now I assumed the pins on the mele were
> wrong, but off xourse it can also be the usb-ttl convertor I got from
> Tom.

The Mele A1000 pinout is, right to left looking from the front, pin 1 is
marked with an arrow on the poard:

1. 3v3 VCC (do not connect)
2. TxD     (output)
3. RxD     (input)
4. GND     (MUST connect to GND on the UART)

which is what is drawn on the picture.

Regards
Henrik