| ??? 06/09/06 13:30 Modified: 06/09/06 13:34 Read: times |
#118094 - Monoflops Responding to: ???'s previous message |
Ganeshan said:
1.Then what is the use of One-Shot? In situations where I need a delayed pulse, what do I do? I have used LS123 in another product where I needed a 1 ms Window after an event. Of course, that was deep inside a board, between two gates, not exposed to any kind of noise. I don't see monoflops as bad as Richard and Erik. I use them by myself from time to time. But you must know its limitations! The first mistake made by users is, to think the monoflop is a digital circuit, like a nand gate or similar. That's wrong, it's an analog working circuit! Why? In a monoflop a cap is charged up to the moment when a certain threshold is reached. Then, a comparator skips and the monoflop pulse ends. The critical point in any monoflop design is the comparator, which must neither show any noise susceptibility nor any drift (long term, temperature, etc.) nor must it be affected by supply voltage changes. Typical monoflops are even suffering form internal noise, means the noise the monoflop produces internally. So, when you look at the pulse lenghts of several monoflop pulses coming out of such a monoflop you will notice deviations from pulse to pulse:  What tells this figure? 98% of all the pulses you can observe will show a deviation from correct value of +-8%, if you use an external timing resistor. (It's +-20%, if you use the internal one!) What this figure does not show is, that the remaining 2% can show totally bizarre deviations from correct value! Yes, some of the pulses can even never occur!! Notice: What's shown in the above figure is only the deviation coming from the part itself, when it's sitting in a perfect and noise free environment!!! But when putting this monoflop into a digital circuit, then the unavoidable ground noise of several hundreds of millivolts can make your monoflop produce pulses of totally unpredictable pulse lengths! Also, the noise on Vcc can make lots of trouble with such a monoflop. Don't forget, the internal comparator of monoflop must skip exactly at the always same value, exact to the millivolt. But that's not what digital chips are designed for. Remember their noise margins. So, to make a monoflop successfully working, you must handle it like an analog chip. You must put it at a quiet place on the board and must filter the inputs and supply voltage. What you have done with your monoflop is a heavy violation of this rule: You put it into digital circuitry, have no sophisticated Vcc filter and connect to its input a 10m long wire without any adequate filtering. You also use the input without Schmitt trigger function, which is absolutely forbidden, and toy arround with the inhibit input. This can make the monoflop behave entirely unpredictable! Kai |
