Re: A10-OLinuXino-LIME board
On Mon, Aug 25, 2014 at 01:46:50AM -0700, Joey Hess wrote:
> Ian Campbell wrote:
> > Why hd-media? The standard netboot images work fine on sunxi AFAIK
> > (testing on cubie{truck,board}).
>
> Board doesn't netboot by default AFAIK, so this would need a serial
> console, which needs a nonstandard cable.
Hello,
I have just looked at the board manual at
https://www.olimex.com/Products/OLinuXino/A10/A10-OLinuXino-LIME/resources/A10-OLinuXino-LIME_manual.pdf
and found the the description of the serial port rather
confusing. By default, the A10 SoC provides a serial port at
3.3V-CMOS level (i.e. 0V for a logical 0 and 3.3V for a logical
1), which is kind of a de-facto-standard for embedded boards
nowadays and for which there is a plethora of cheap (~3 EUR)
USB-to-serial cables easily available (most of them appear to
be based on the PL2303HX USB-to-serial chipset).
The board manual states on page 19 "Note that the levels at
board's UART0 are in CMOS level and you would need a MAX232
convertor to bring them to TTL one!". The first half of this
sentence makes sense, as it is what the SoC provides by default,
but the second part does not make sense. A MAX232 converts TTL
serial levels (0V for a logical 0, 5V for a logical 1) to RS232
levels (between +3 and +15V for a logical 0 and between -3V and
-15V for a logical 1) and is used for interfacing 5V-TTL circuits
to a classic 9-pin/25-pin PC serial port with RS232 levels. It
does neither work reliably with 3.3V signals on the SoC side, nor
does it output TTL (i.e. 5V positive logic) on the other side.
There is a MAX3232, which is the 3.3V-CMOS-compatible version of
the MAX232, but like the MAX232 it provides negative-logic
high-voltage RS232 levels on the other side an not TTL levels, so
I think that the information in the manual is simply wrong.
For having a serial console on the board and being able to
netboot from u-boot, buying a standard 3.3V-CMOS-serial-to-USB
adapter should solve your issues. As a side note: you need only
the RX, TX and GND connections from the cable. Some of these
cables come with an additional connector on which there is a 5V
supply voltage from the USB port for powering external devices,
even on cables which are designed for 3.3V signal levels. Never
connect this 5V line anywhere to your board; depending on the
board circuitry you might fry the board otherwise.
Kind Regards,
Karsten
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