kasper

Hi

I have a D type flip flop that I want to reset about 4nsec after the Q output goes high (sort of a 'one shot' circuit). Could someone give me a reliable circuit for that. The reset is active high and the flip flop comes with complementary outputs.

Thanks

David Bridgen

An RC from the Q to the reset pin. (R from Q to reset. C from reset to ground.)

The reset will go high once the C has charged up from the Q through the R.

kasper

Thanks David; so the 4 nsec time in the unstable state can be set with this RC; but through what path the capacitor will discharge back to zero volts so that the D flip flop is ready for the data at next clock cycle.

David Bridgen

When the flip-flp is set the Q goes high, and 4ns later so does the reset pin.
(The C was charged from the Q through the R.)

The Q then goes low, so the C discharges through the R and the Q.

Voila.

ljcox

Why use a flip flop?

If you want a 4 ns pulse, this could be done by an RC differentiator followed by a Schmitt Trigger .

Len

tavib

First of all the 4ns is a very short pulse, you must use a fast response time logic if you want a cvasi-rectangular pulse. For example the HC series have 6-8ns rise/fall time and this series is faster than LS-TTL so you must use AC series with 2-3ns rise/fall time. In this cases simply use propagation delay through circuits to generate such short pulses.