A few days ago, a post on HackADay.com featured a Vetinari clock. Though I have yet to read any of the Discworld series, I found the concept so intriguing that I began to consider my own build.

Not wanting to repeat the work of others, only with a different CPU, I set out to find another way. Eventually, I arrived at a LFSR (Linear Feedback Shift Register) based design. Here is a link to a simulated 8-bit version. 

In the simulation, the LFSR has a few extra gates to force some logic 1s in to the shift register at start-up. The clock input to the counter is running at 128 Hz. When the counter reaches 255, its' carry ouput goes high, first causing the LFSR to advance to the next value, then loading the counter with the LFSR's new value. Every 255 carry events, the LFSR is back at its' original state and the counter has counted 32,640 clock cycles.

Rummaging my parts stash, I found a 74322 8-bit shift register and 7486 quad XOR to build the LFSR. No 8-bit counter to be found however, so a pair of cascded '161s will have to do. A little extra logic help from a 7400 quad NAND, and it's time to build. Time? What kind of prototype would this be if it didn't have a 555 timer? It's a hack, but the 555 is running at 125 Hz and I didn't have to go look for the signal generator.

If we scale this up to 16 bits, it could run directly from a 32768 Hz oscillator. It would have the correct time every 18-ish hours. A wonky timebase, a 7 segment display made from LED filaments, this might be a new project.