[time-nuts] BBB DDMTD - analysis
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Thu Oct 30 14:45:24 UTC 2014
Lots more pictures and data uploaded here:
In an effort to understand which component was responsible for my ~17us
spikes I decided to go back to basics with just a single DFlop (AC74) on
a breadboard; no BBB, just a couple of oscillators driving the data and
clock pins (the microcrystal and 8663 in this case). I think I can hear
Bob shaking his head at using a breadboard :) but the point was to
change the parameters (even if it was for the worse) to determine what
was causing the problem.
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.
What surprised me about these traces is not that strange stuff was
happening at all, but just how long an interval it was happening for.
I'd expected, say, tens of clock cycles (~1us maybe), but here the dflop
is still reacting over 100us later. The period where the output was
floating was just over 50us long.
The next few traces (DFlop-sync-*) show the output synchronised through
the second DFlop of the AC74. The first few clearly show glitching bang
on the 17us mark, revealing that the data I'd seen wasn't because of
quantisation but because glitches were specifically occuring at those
points. It also nicely ruled out the BBB as this wasn't connected.
The last few DFlop-sync-* traces show a variety of edge transition
cases. What is becoming clear is that the set of transitions near an
edge are _not_ a nice standard distribution as you might expect from
simulation, so this is going to make the edge detection algorithm
After messing around with the AC dflop, I decided to try swapping to a
slower part to see what impact this would have. I switched to an HC595
shift register, in part so I could also rule out my mess of wires
between the sampling and synchronising flip flop as being the cause of
the problem. The last couple of traces show an HC595 seeing exactly the
same type of glitches as the DFlop.
Finally, I hooked the BBB back up and tried quite a few different
combinations of parts and clocking techniques to see what impact they
may have. I've included plots of the edge transition distribution of the
AC74 dflop, HC595 shift register and AHC595 shift register for comparison.
Each of the profiles are slightly different, but they _all_ show
glitching at ~17us increments. The final surprising bit is just how
consistent this number is given the wide variety of different setups
I've tried. I've changed practically most of the setup at some point or
another and despite how hopeless by hardware layout has been, the
transitions have always been occuring within one or two clock cycles
every time (169-172 sample clocks).
To wrap up, despite the glitching, I managed to get noise down to reach
about 7E-11 @ 1s which is pretty hopeful given it was botched up on a
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