by antto » Sep 24th, '17, 10:44
yup, that's one of the discouraging aspects of the yocto cpu
the fact that the PCB is black also doesn't help much
there is a 10pin (5x2) header near the cpu, which smells like avr ISP connector, but it's not labelled, so who knows
other than that, you could check the available USARTs on the chip, and try to see if those pins go onto traces somewhere on the PCB (that would be easiest if you have a schematic and/or board layout files to look at)
if you find an USART module with both TX and RX pins not used (and not connected to anything on the PCB) then i can think of a little dirty hack:
use a an extra 40-pin IC socket, plug the CPU onto it, bend the pins where the RX+TX are on that socket sideways and solder wires to them, then plug that whole thing into the socket on the PCB
you'd also need to connect another wire to a ground point (preferably taken from somewhere in the digital section of the circuit)
now, you have three wires TX RX GND, you can use a USART<->RS232 converter box, or a USART<->USB converter box
don't connect the USART directly to a RS232 port on a computer because RS232 is not 5V
if you actually only need to print out debug information - you only need the TX pin (and GND), but having both TX and RX available on the same USART module is a better option, because then that can be used for a serial bootloader (because MIDI is damn slow)