The v5.0 variant boards arrived today and I built one with blue LEDs. The bad news is that blue 7 segment LEDs are hopelessly dim. Even blasting them with on the order of 4.3 volts doesn't make them any brighter (it just makes them a little warmer). To make matters worse, I discovered to my horror that the display module for the 10th-of-a-second digit is wrong, and that nobody makes a blue one that size with the correct pinout. So it's back to red. I'll build one of those (or rip the displays out of this one and replace them) later.

That said, the multiplexing system works beautifully. The one firmware tweak I needed to make was a short delay (a short _NOP() busy-loop) between turning off the anodes (this would really only happen at maximum brightness - at reduced brightness they would have been off already) and shifting to the next cathode and then presenting the new anode values. This is because the high-side buffer chip has a turn-off spec of 2 µs. Before this there was some ghosting in the display at maximum brightness. Other than that (and the fact that blue LEDs suck), the firmware testing I did in advance paid off handsomely and the clock is indistinguishable from the previous versions.

The next variant is already designed, but ordering it had to wait for this validation. The big change in that one is to replace the LDO and buck converter with a single dual-buck chip - the MP2149GJ. I'm kinda torn about that, though. It costs more than the combination of PAM2305 and LDO, but it's a net BOM reduction (and it's only one active part instead of two).

For the purposes of the Hackaday Prize, the BOM listed is the v5.0 design. If the clock makes it to the next round of the Best Product category, the v5.0 design will be the one of which 3 copies will be submitted as prototypes.