Naturally, if a kernel (like the latest one) refuses to boot and does not even get to turn on the screen, the only way to debug the reason is to use the serial console.
With the indications and the useful blogpost (for MK808) of fellow developer Omegamoon, I set out to find the two pins necessary to watch in my PC the boot sequence, that is:
- TXD: the pin where the CPU outputs the console text
- Ground: the necessary reference for the electrical levels in TXD
These had to be connected to the RXD and GND pins, respectively, of a hardware serial-to-USB module, that is then plugged to a PC's USB port. If anybody wonders about the CPU's RXD pin, it is not necessary unless you want to send commands to the stick once it's booted.
In the PC I myself use Ubuntu's CuteCom console app, connecting to serial port /dev/ttyUSB0 at 115200 bauds 8 bits data, 1 bit stop, no parity, and no handshake.
The way in which I found the RK3066's TXD pin was by trial and error. But since I saw there were small circular pads around the same area where Omegamoon had found his stick's TXD/RXD, I supposed mine should be close.
Hence, I first soldered the ground cable to the micro USB port on top of the RK3066 CPU and started very carefully making the serial-to-USB module's RXD pin's cable touch the different pads while powering on the stick, until I found the one that was transmitting data: TXD and proceeded to solder the cable and make the smallest possible hole in the Measy U2C's enclosure to get the two cables out of it while closed.
For reference, these are the places where the cables to the serial-to-USB are soldered
|TXD pad of RK3066 (to RXD on serial-to-USB) and GROUND on the Micro USB connector|
The TXD pad looks tiny but with a bit of care and a little bit of solder tin on the cable tip and the solder tip, you'll get good results.