Yay! I caught the leap-second on my new, more sophisticated (alas still on a breadboard) GPS clock. Scope creep got the better of me, so I haven’t yet soldered up a permanent version, but it is coming.
The clock uses a ublox Neo6M model for time reference, but serial connection is too slow (about 200ms latency). Instead, I soldered a wire onto the PPS and use it to drive an interrupt on ATmega328p clocked at 3.86 MHz. The PPS time compares very favorably with the one from my Trimble Thunderbolt (the ublox is actually ahead of the Trimble – though I haven’t configured either). The display updates on the main loop which runs 45-50 KHz (physically verified with my HP counter), ergo within a few milliseconds.
I added a menu-driven UI controlled by four pushbuttons (lower-right of the breadboard) — it allows me to tweak time and display settings. The photo to the right shows the contrast setting in action. Also the “lp” over the seconds indicates that a leap-second was pending.