Logic Analysers

[Adrian Graham]

On 04/02/2017 05:40, "dwight" <dkelvey at hotmail.com> wrote:

> All the glitches are at the beginning of the ALE. There is nothing
> there that has any meaning. Things are changing at this time. Not
> every thing changes at the same rate. That is why they have an
> ALE to mark when the address is good. When high, the circuit address
> latch is open. When ALE goes low, it captures the address.
> You really should be looking at the processor timing diagram and
> understand what you are looking at.

I'm doing that right now as it happens, I've got a dead tree edition of the
MCS85 User Manual that has all the timings in and I'm trying to match them
with what the CPU is doing. What I've been thinking is happening is because
the traces I'm seeing don't match exactly on screen then there's a problem
but of course as long as the transitions are happening within the right time
frame there isn't a problem.

> 
> Technically a glitch at the beginning of the ALE can last until
> some nanoseconds before the falling edge and the circuit would
> work fine. These glitches are much shorter than the ALE and
> clearly not an issue.

So I'm looking too deeply, OK.

Back to the books for me then :)

A

> ________________________________
> From: cctalk <cctalk-bounces at classiccmp.org> on behalf of Jon Elson
> <elson at pico-systems.com>
> Sent: Friday, February 3, 2017 6:36:27 PM
> To: General at classiccmp.org; Discussion@
> Subject: Re: Logic Analysers
> 
> 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.
> 
> Jon
> 

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
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