1. Timing circuits Monostable multivibrators (one-shots) –Digital storage circuit with only 1 stable state –Temporarily driven into a transient state by a 0  1 or 1  0 transition on their inputs –The duration of the transient state is controlled by the timing constant of an external RC network –Can be retriggerable  transient timing cycle is restarted each time the input experiences a transition –Non-retriggerable  after initiation of a timing cycle, further changes in the inputs are ignored until completion Astable multivibrators (oscillators) –Generate a sequence of pulses

2. Timing circuits The 555 integrated circuit timer –Can be used in both monostable or astable mode, depending on how connected Vtrigger < 1/3Vcc sets the RS FF Vthreshold > 2/3Vcc resets the RS FF 5V < Vcc <15V

3. Timing circuits The 555 integrated circuit timer in monostable mode –Stable state: Q=0 (Q’=1), C is discharged (why?) –Pulse U on TRIG sets Q=1 –C charges through R A –Until V C =V THRESH >2/3V CC –Ignores new TRIG pulses until cycle complete (why?) –T pulse  1.1R A C

4. 555 in astable mode –Q=1 (Q’=0), C charges through R A +R B –When V C >2/3V CC, the R input of the internal FF becomes 1 –The FF flips, Q=0 (Q’=1), transistor Q1 opens, C discharges through R B –When V C <1/3V CC, the S input of the internal FF becomes 1 –The FF flops, Q=1 (Q’=0), transistor Q1 closes, C charges through R A +R B –t H  0.69(R A +R B )C –t L  0.69R B C –T  0.693(R A +2R B )C

5. Rapid prototyping of sequential circuits Use PROMs for the logic –Easy to re-program –Directly implement function –No need to optimize

6. Rapid prototyping of sequential circuits (c) implementation (a) State table (b) PROM truth table

7. Rapid prototyping of sequential circuits Prime number sequencer –What should be the contents of the non-prime locations?