Thanks to jamez70's, we now know where the serial pins are.How to find the UART
Here is where the UART pins sits on the back of the motherboard.
(Courtesy of ifixit photo and jamez70's illustration)
The pins from 1-4 are:
RX TX GND GND (where #4 is marked)
From probing, we can detect the 1V8 level signal and the baud rate of 115200.
(Courtesy of jamez70)
If you want to hook this up, I suggest you solder on some wires to the pads and run the wires to the front, making life easier to re-assemble the KF.
I suggest you get something like the Breakout Board for FT232RL USB to Serial for hooking this up. Note that the KF is 1V8, so when you want to connect this unit (default 3V3), you may want to use a level shifter, or simply find the VCC on the KF and loop that back that through the VCCIO on the Sparkfun-converter. Sending 3V3 directly into the KF could possibly seriously hurt it!Details
If you want to construct a simple voltage divider, simply get two resistors and get your soldering gun out. According to Vout = Vin * Rb/(Ra+Rb)
Voltage divider circuit (courtesy of calculatoredge.com)
- Vin: 3V3
- Vout: 1V8
- Ra: 10 kOhm
- Rb: 12 kOhm
If you need more UART details, here are some tips:
- Baud: 115200
- Bits: 8
- Parity: None
- Stop Bits: 1
In the work to get a booting 3.0 kernel on the KF we need some low level access to debug since the USB initializes way later in the kernel sequence. I did some research on the OMAP4430 (the CPU in our dear KF) to try and find out where the UART interface might be accessed.Background
Looking at the datasheet we get a feeling of all the ball points ("pins") on the CPU. (Observe that this is seen from under the CPU)
OMAP4430 ball bottom layout
By then looking at the UART table TX pin numbers I try to get a feel where they might be located on the chip.
UART 1, 2, 3, 4 table
Finally, I cross referenced the UART table with TX pin and physical position on the board. The colors represent UART 1 TX, UART 2 TX and UART 4 TX. UART 3 doesn't seem likely to be mapped.
UART 1, 2 and 4 TX pin guesses
Observe that these are all estimates, but should narrow the search down (unless they're mapped somewhere way outside of the CPU area that is). I haven't had a chance of testing any of my guesses yet, but I thought I'd at least post my thoughts to get the ball running.