Digital Monostable?

Hi,

1) Is there any equivalent to monostable in fully digital electronics
(logic gates)? I am trying to trigger a pulse (a set width) with a very
narrow recovery time (in the range of 100ns or less) before the next.
74LS221 seems to work although getting weird pulses (very short
retrigger pulses) occasionally.

2) And also whats the difference between the normal monostable and the
precision ones? As the name suggested they'd be more precise
triggering?

3) Would 74HCT version will be more suitable although I read somewhere
they are slower than the ls version.


Thanks,
John

 

This is not a problem with the device but rather power supply decoupling,
glitches on the clock signal, a race condition, or noise on the wiring.
Monostables work reliably when properly fed.

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Sam's right. Any noise on the power buses is going to be bad news for
the monostable. That's because a monostable works by comparing a
rising ramp voltage with some logic threshold. There's no problem at
first, because the ramp is down near zero volts and the threshold is
like 1/2 to 2/3 Vcc, But as the ramp rises, the monostable keeps
trying to compare 1, 2, 2,.5, 2.6, 2.7 volts with the current
threshold. As the time interval draws to a close, it's trying to
compare smaller and smaller differences. Now the other fully-digital
chips won't be bothered by a volt of noise, but this comparator will
jitter all over the place, in proportion to the jitter on Vcc.

Try putting a 100 ohm resistor in series with the +5 volts going to the
monostable and bypass that to ground with a 10uF capacitor PLUS a 0.1uF
ceramic. Make sure the timing resistor and capacitor are fed off this
decoupled Vcc source too!..

 

And if you need somewhat more stable delays, there are "delay line"
IC's, basically 74HC14 inverters with little resistors or inductors
between the inverters. Kinda kludgey but the kludges are hidden under
an epoxy coating.

 

All monostables in that region I'm aware of, use RC for the timing.
(Although that R is not necessarily an external one.)

Sam mentioned the necessity of powerline decoupling already. Another cause
of spourious pulses are distorted trigger signals. Long lines (wires) or
component mismatch are known for that.

Generally speaking you should expect more precise timing. That's to say the
pulse duration should be less depended on the IC and more or only on the
external components. But the slogan "precision" can be interpreted in more
ways. So the real properties should be extracted from the datasheets.

Once more the datasheet(s) should tell you.

petrus bitbyter