When I designed the clock, I neglected to include space for some sort of jtag connector. I left unlabeled through-hole solder pads on the board so that I could program it, but they had to be soldered/unsoldered each time I wanted to take the clock out of service and reprogram it. I finally realized that I could just take the same connector I am using for jtag on the oscilloscope and just epoxy it on one of the edges of the board and run little wires to where they had to go and that would be elegant. I did so and it reprograms properly.
tiniest bit of solder
The oled display can handle up to 18V, so I designed the DC-DC converter power supply to deliver 16.3V to it. This turns out to run the display too bright and too warm (the display has an image burned in now as a result). So I calculated that by replacing a 120k feedback resistor with a 100k resistor, I could get it down to 13.8V. While replacing the resistor, I noticed a stray speck of solder that was between another resistor and a trace, so I heated it up a bit to see if it was supposed to be there (if there was a trace between them on the board), but I couldn’t tell. When I plugged it back in, the image on the display had disappeared (it was there, but it was extremely dim). I assumed it was the resistor I had just replaced and not the speck of solder, so I tried soldering/unsoldering it and going back to the original resistance a couple times unsuccessfully.
Finally, I checked the schematic (RTFM?) – there, I noticed that the shutdown pin needed to be high for the DC-DC converter to be active. It was measuring low, so I checked the source code and I had never written anything to drive that pin high. This implied the blob of solder had done that for me from the first time I soldered it together, nearly a year and a half ago. The splork of solder had shorted the pin on the microcontroller that controlled the DC-DC converter’s shutdown pin to something high. Now that it wasn’t being shorted high, I needed to write some “turn this pin high on startup” code and program it (with the fancy new jtag connector from above). I did so, but the display still didn’t come on. After some frustrating time trying various things to get the display to come back on, it dawned on me that it was the 16.3V display voltage line that had been shorted to the microcontroller pin and that might have damaged the ability of the microcontroller to drive that pin high. I scraped part of that trace off and connected it to another spare i/o pin on the microcontroller and changed the code correspondingly and now it works fine.