[time-nuts] BBB DDMTD - analysis

[Simon Marsh]

Lots more pictures and data uploaded here:
https://drive.google.com/folderview?id=0BzvFGRfj4aFkMFBtNWFSZVBKWkk&usp=sharing

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 
interesting.

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 
breadboard.

Cheers


Simon