[Arm-netbook] Hacking the Mele A1000 - ultra beginner guide

[Henrik Nordström]

fre 2012-03-23 klockan 10:34 +0800 skrev julien at forgeat.name:
> Received the USB TTL Serial/UART module :-)

Great!

> As Alejandro stated, the 2.54mm cable connectors do not fit, we can at 
> most fir two out of 4, maybe 3 but that would be risking breaking 
> something ^^

Yes..

you can strip the middle cable connector from it's plastic to cut down
size a bit, which may allow you to fit GND, Rx, Tx

Also pay attention to order of Rx and Tx. These needs to be crossed. Rx
on your cable to Tx on mele, Tx on cable to Rx on mele.

> But then, I have been thinking:
>   (1) the 3.3V Vcc is apparently not always needed (is it only if the 
> cables are very long?)

If you have a fixed voltage (including voltage selected by jumper)
serial cable then Vcc is not needed.

The 3.3V pin in the serial port is for serial cables with variable
voltage level. So you should leave this unconnected in most cases.

The only case when you should connect the serial port 3.3V to the serial
cable is if you use a buffered/isolated cable with variable voltage
level and requires a VREF/VIO reference power driving the I/O logics at
the right level for the board.

Almos all cables have fixed I/O voltage level (single, or selected by
jumper), and on these you should leave the VCC unconnected. If the cable
do have a VCC pin then you MUST NOT connect VCC. The VCC pin provided in
the cable is for powering the device you connect to.

>   (2) 2 cables fit in the connector
>   (3) We should be able to get ground from somewhere else on the board

Yes.

> So is it dangerous the connect Rx and Tx on the UART port, Ground on 
> the USB port and not to connect Vcc (or why not, get Vcc from the USB 
> port)?

ground should work.

But DO NOT connect VCC from there. You will likely destroy something.
* Your cable do not have a VREF/VIO connection
* VCC of your cable is 3.3V. USB uses 5V.

Connecting VCC of your cable to USB you short the 3.3V serial cable
power output to a 5V power usb power output, and BAD things are likely
to result, possibly (but probably not) involving magic blue smoke
leaving the A10 CPU or usb serial chip if not 5V tolerant.

Regards
Henrik