AVR HV UPDI FTW!
We have been working a lot with attiny816 and attiny1616 chips (https://www.adafruit.com/search?q=attiny) lately, as for our seesaw boards. We often must program them with a CP2102-based breakout (https://www.adafruit.com/product/5335) with a 4.7K resistor soldered between the RX and TX pins. But we are longing for a nicer programmer. Perhaps one that can select 3V or 5V power and logic? And activity LED? and 12V High Voltage programming support if we accidentally set the HV-Only fuse? We looked around and found this charming open-source hardware design (https://github.com/wagiminator/AVR-Programmer/tree/master/SerialUPDI_HV_Programmer), and it inspired us to make something similar.
Instead of a USB A plug, going with USB C. We've used the CH9102F (https://www.adafruit.com/product/5568) but never the CH340E, but it seems like it will do the job here. The classic 3.3V AP2112K LDO is kept for power, and the MT3806 booster looks neat, so that goes on too… plus the 3/5V selection switch! The most significant change is going with a COS4561 analog switch for the 12V/UART signal swapping and a JST SH 3-pin for connecting a quick wire harness (https://www.adafruit.com/product/5755). It fits nicely on a single-sided PCB; most of the space goes to the booster and analog switch, so we'll make a non-HV version.