Logic Analysers

Jon Elson elson at pico-systems.com
Fri Feb 3 20:36:27 CST 2017

On 02/03/2017 04:34 PM, Adrian Graham wrote:
> On 03/02/2017 19:43, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:
>> But that's why I said 'about'. I am doing order-of-magnitude calculations,
>> not trying to design a delay line. I would estimate that between adjacent
>> ICs on the same board you'd get a delay measured in 10's or 100's of
>> picoseconds. That sort of order. So a 25MHz logic analyser, with an
>> effective time resolution of 40ns (if that) is not going to show it.
>> There is no way you're going to get delays of 40ns between adjacent
>> ICs on any reasonable PCB.
> This is the sort of thing I mean:
> http://www.binarydinosaurs.co.uk/STCExecutelA1checking.jpg
> Watching the A1 address line (no triggers just sampling 6 points) and a
> pulse appears at ROM4 on the falling edge of the ALE signal but not the
> other 3 ROMs or the LS373 flip-flop that's demultiplexing the AD1 pin of the
> 8085. While I was thinking about the possibility of propagation delay I
> noticed this one:
> http://www.binarydinosaurs.co.uk/STCExecutelA1checking2.jpg
> Pulse missing from ROM3.
First pic, pulses are missing from ROMs 1-3, seen on ROM4.  
But, those pulses on ROM4 are really narrow, and may be 
noise, or very narrow glitches.  Any time you see really 
narrow glitches, especially when they are one LA sample 
wide, you have no idea what they actually look like.  The LA 
detects that the pulse was there at the instant it sampled 
it, but you don't know whether it was 5 ns wide, or 70 ns 
wide (with a 40 ns sampling period).  You also don't know 
whether they were full-amplitude pulses or runts that just 
barely crossed the logic threshold of the analyzer.  So, I'm 
not sure what you've shown there actually represents a 
problem or not. Especially on the 2nd picture, the pulses 
you have highlighted really look like a single sample wide, 
and if the logic levels of the analyzer are not exactly the 
same, or other slight deviation, it could have missed a 
narrow glitch.  Anyway, on old 8-bit micro gear, there may 
be plenty of narrow glitches in the 40 ns range, but the 
operation of the chips is most likely NOT going to depend on 
the circuits responding to such glitches.  I think you are 
chasing your tail about these things, and missing a real 
malfunction that is not related to this.  Could be EPROM 
bits that have faded, one shot capacitors that have changed 
value or something.


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