Stable start/stop oscillator - for video character generators

Start/stop oscillators are indispensible in video interface circuits. Such oscillators have to be synchronised with differentiated character clock pulses and produce 7 ... 12 pulses between character clocks. There are two aspects which are important to note here: • The oscillator must start producing pulses after a delay of about 15 ns. This prevents the first pulse (the output signal) from coinciding with the positive-going edge of the trigger signal. • The oscillator must stop as soon as the control signal goes low again. The oscillator shown in the circuit diagram meets both of the above requirements. It starts after a slight delay whenever the input signal goes high and stops immediately the input signal reverts to logic zero. R1,R2 = 560 Q. 4k7 C1 = 20 .. . 80 p N1 = "A 74LS00 N2 = "A 74LSO4 stable start stop oscillator for video character generators 14