Weird bug on the servo board

A project log for 6502sbc Robot

65C02-based SBC with a discreet-part servo controller and ADC intended as a intermediate-skill learning tool.

CarbonCarbon 08/13/2014 at 18:390 Comments

Still working on the servo board.

I've run into an interesting bug. The data inputs from the bus are doubled buffered so the controller doesn't swap out the compare value part way through a cycle and cause glitches in the output waveform. The buffers are two eight bit registers, one clocked on the data from the expansion bus and the other clocked on the counter reset conditions.

The first register, the one that holds data from the expansion bus, clocks in data exactly twice and then refuses to clock in new data (or at least refuses to show it on the outputs) for at least the next few minutes (that's as long as I've tested it).

I've verified with a logic analyser that the clock and data input signals appear as expected, and the problem exists with both channels on the board (so I don't think it's a bad part). It's repeatable as well; I power cycle the board and the same thing happens every time.

I'm stumped; anybody have any ideas or things to try?