Motorola databook would have it and they weren't rare either. I've run them down to 100khz, the timing of the signals moves a bit as at those slow speeds the propagation delay is not significant anymore relative to the cpu timing. I did that once with a 68k with apparent normal (and very slow) behavour. With rare expection most CPU underclock to the extreme and most will overclock by some. The exceptions are any of the 8080/8085/z80 and similar cpus that use dynamic memory cells and are NOT static. Though I've taken them to very low clock speeds as well though not to full stop. I'ts rare to see a data sheet specify a minimum clock that isn't very slow or at least the vendors testing minima. Allison