,It's been a while since the last update. My last experiments with the blue CN2596 boards revealed a problem with the reliability of the simulated serial interface when less than 3 digits of the LED display were active. The old board suffered some "bad stress", and there was at least a tiny chance that the problem had to do with that.

I now received a couple of revised DCDC boards with green PCBs marked CN2596-2. The new boards have unpopulated pads for a ICP connector (Vss, SWIM, NRST, GND), and I retrofitted a pin header to it. After flashing the DCDC code it showed exactly the same behavior as the blue DCDC board.

It must have something to do with the PC.7 interrupt, and I still hope that it's a bug in the 7S multiplex code, and that it has nothing to do with the lack of capacitors in the power supply, or current limiting resistors for the LED display.

By the way, the parent project made some good progress.

EDIT1: unfortunately it has something to do with the 7S-LED display. When I remove it (on the blue board it's socketed) the display contents doesn't influence the serial communication at all. When I plug it back in, the communication problems appear. I'm now looking for a workaround.

EDIT2: actually it's a no-brainer: the communication through PC7, which normally drives the DP segment, works if at least 2 other segments are lit (then the port sinking the common anode limits the current sufficiently). Sorry, no sign (-) while the communication is active. For a power supply that's easy.

EDIT3: I hacked the LED MPX routine to mitigate the handicaps inflicted by the abysmal design. Of course, it would be safest to disable the LED display, but no risk no fun, right?