[time-nuts] BeagleBone Black DDMTD update
kb8tq at n1k.org
Wed Oct 29 19:10:27 EDT 2014
In the case of a 1 ps aperture on the flip flop, and a 100 fs delta between samples, there are two simple things that might happen:
1) You get random garbage for 10 counts.
2) The flip flop “hangs up” for 10 counts.
To know which one you are going to get, you would need a pretty good model of the flip flop.
If your reference signal has sub-harmonics in it, (it’s 5 MHz doubled to 10 for instance) then they will give you a “double edge” when viewed on a scope. A ten or twenty ps delta is not at all unusual in that case.
If your edge rates are in the ns / volt range ( = it’s fast logic), and your ground plane is more than 30 mils from your leads, you will get things coupling here and there. If ground and power are not planes, you will get bounce. All of that will create odd outcomes on the flip flop.
If your bypassing is not short lead, and reasonable sized, the power pin will “drop” when an edge goes out. That’s even more true if you have a heavy load. When that happens, odd internal things can / will happen in the flip flop.
Modern gates are designed to be surface mounted on a multi layer PC board with chip bypass. The further you get away from that “model” the more likely you are to have less than perfect behavior. You are after *very* good behavior ….
> On Oct 29, 2014, at 6:50 PM, Simon Marsh <subscriptions at burble.com> wrote:
> On 29/10/2014 22:22, Bob Camp wrote:
>> It is not at all unusual for signals to be re-clocked when going into a micro. Often the documentation on this process is somewhere between vague and non-exsistant.
> Yes, luckily the Sitra TRM has a nice clear diagram for the mechanism I use and the signals are re-clocked twice on their way in to the BBB (@100mhz for the OCP bus then @200mhz for the PRU).
> The timing at the BBB is not critical though, the signal is captured in the sampler @ 10mhz and the BBB has a shade under 100ns to read it before the next sample comes in. In initial testing, I had the BBB continuously sampling and had no problems keeping up @10mhz (I actually had to slow it down to make it work).
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