Hello all,
I have got a problem with a digital circuit. I got a signal from a comperator, which changes from 0 to 1 and back. Now I want to catch the first high signal and hold it until infinity. I try to use a RS-FF with R = 0 and S = 0,1,0,1,... (my input-signal) , but I got, of course, an undefind state until the first high comes. How can I get a zero in the first moment and not an unknown state?
Thanks a lot!
Irina
Use a "D" flop. Connect the "D" input high (5V) and connect your signal to the clock input. Take the output on the "Q." When the first positive edge of your signal appears, the "D" flop will change state to high out where it will remain. The flop may be reset by using the reset input and should also reset on power up.
It seems to me that an RS latch is just want you want for this application.
If I understand you correctly, by saying that R=0 and S=0 is an undefined state, you are implying active-low logic. If that's the case, you want R = 1 for your circuit to operate. Your circuit can't latch if it's perpetually reset.
But you do want a one-time reset when you set the latch into action. The obvious solition is to use a momentary reset switch to reset it. Another option is a high impedance pull-up resistor and a large value capacitor on the reset pin. Maybe 10K and 1 uF. That way, on ititial power-up, the reset pin will be held low for roughly one time-constant. You get a one-time reset pulse, and then the latch is ready to catch the first (active low ?) pulse on the set pin.
--
View in Courier:
IN----------A
NOR Y--+
+--B | Vcc
| | |
| A--+ [0.1µF]
+--Y NOR |
| B-------+
| |
+-->OUT [100K]
|
GND
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