> From: Simon Marsh <subscripti...@burble.com> > Subject: Re: [time-nuts] BBB DDMTD - analysis > Message-ID: <54524f04.7060...@burble.com>
> The first few pics (DFlop-unsync-floating-*) show > the Q output, which was unconnected to anything > other than the oscilloscope. They show a few glitches > at the edge then a load of funky stuff going on later. > Connecting Q to an input pin (DFlop-unsync-connected) > cleared the problem suggesting that in this period, the DFlop > output wasn't being driven and Q had been left floating. I find that description really confusing. Can you draw up a schematic? If you connected the Q output to an input later, what had been driving that input previously? Was the input driven by an oscillator, but you disconnected the oscillator and connected the output to input instead? Or the input was floating prior to connecting the output to the input? > the DFlop output wasn't being driven and > Q had been left floating If you have a standard 74AC74 the output never floats, it is always driven as far as I know. Even if the output had been floating, if it was truly not driven you would have seen it as low due to the input impedance of the scope pulling down the output. On the other hand, if you leave a CMOS input floating you get all kinds of weird behavior on the output, every little disturbance can change the input charge enough to toggle the output. Are you sure you weren't just seeing oscillation because the input was floating? Are CLR_ and PRE_ pulled up always? Are the input and clk pins always driven by the two oscillators? Have you looked at the two oscillator outputs at the beginning of each run to see how close the edges are to each other? The timing relationship will of course drift (unless the two oscillators get injection locked to each other), but that will at least give you an idea of how likely you are to hit setup and hold problems at the beginning. You mentioned 17us periods in the glitching, have you checked the frequency offset between your two oscillators? You aren't just seeing the beat frequency? 17us is just a hair under 59kHz, that seems really far off for a decent OCXO, just wondered if you had ruled it out. -- Chris Caudle _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.