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rising-falling edges

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Bart

Jan 1, 1970
0
In my projects with flip flops I notice my led's have tiny off's or on's
that are out of rythm with the function or counting of my circuit. I set my
555 astable to about 1 herz so I can visually watch the sequencing of my
led's and my O'scope shows a very nice clean rythmic square wave. My
circuits are simple with two chips, both dual flip-flops and some led's. Now
I'm trying circuits with counters, for example a 74LS90 and some led's and
I'm noticing the same tiny "flickers" out of rythm. I read about
rising-falling edges as triggers, might this be a factor on what I'm seeing?

Any help is appreciated,
Bart
 
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Bart

Jan 1, 1970
0
I found this:
"These are ripple counters so beware that glitches may occur in any logic
gate systems connected to their outputs due to the slight delay before the
later counter outputs respond to a clock pulse. "

Should I stay away from "ripple" IC's..... Is synchronous a better choice?
Is there "glitches" in asynchronous?

What would a "Rule of Thumb" be on when to use "ripple", "synchronous", and
"asynchronous"?

Still learning,
Bart
 
B

Bob Monsen

Jan 1, 1970
0
In my projects with flip flops I notice my led's have tiny off's or on's
that are out of rythm with the function or counting of my circuit. I set my
555 astable to about 1 herz so I can visually watch the sequencing of my
led's and my O'scope shows a very nice clean rythmic square wave. My
circuits are simple with two chips, both dual flip-flops and some led's. Now
I'm trying circuits with counters, for example a 74LS90 and some led's and
I'm noticing the same tiny "flickers" out of rythm. I read about
rising-falling edges as triggers, might this be a factor on what I'm seeing?

It is hard to say what you are seeing, because you haven't described
the circuit, or what parts you are looking at. However, any change should
be on either the rising or falling edge of the oscillator, no matter how
you have things hooked up. If there is additional flickering at other
times, something is wrong.

--
Regards,
Bob Monsen

Strange as it may seem, the power of mathematics rests on its evasion of all
unnecessary thought, and on its wonderful saving of mental operations.
- Ernst Mach
 
A

Andrew Holme

Jan 1, 1970
0
Bart said:
In my projects with flip flops I notice my led's have tiny off's or
on's that are out of rythm with the function or counting of my
circuit. I set my 555 astable to about 1 herz so I can visually
watch the sequencing of my led's and my O'scope shows a very nice
clean rythmic square wave. My circuits are simple with two chips,
both dual flip-flops and some led's. Now I'm trying circuits with
counters, for example a 74LS90 and some led's and I'm noticing the
same tiny "flickers" out of rythm. I read about rising-falling edges
as triggers, might this be a factor on what I'm seeing?

Any help is appreciated,
Bart

The glitches produced by LS TTL ripple counters are not visible on an LED -
they're only nano-seconds wide.

Do you have resistors in series with your LEDs?

If you turn-up the clocking speed, and look at it on your 'scope, you'll see
that the output high voltage of a 74LS90 is not quite the same for all
states in the counting sequence. This is allowed, as long as it always
exceeds the minimum stated on the datasheet, and could be visible as a small
change in brightness.
 
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