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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

Reply to
cooldude
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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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Reply to
Sam Goldwasser

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!..

Reply to
Ancient_Hacker

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.

Reply to
Ancient_Hacker

"cooldude" schreef in bericht news: snipped-for-privacy@75g2000cwc.googlegroups.com...

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

Reply to
petrus bitbyter

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