Timing circuits

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 5V < Vcc <15V Vthreshold > 2/3Vcc resets the RS FF Vtrigger < 1/3Vcc sets the RS FF

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 RA • Until VC=VTHRESH>2/3VCC • Ignores new TRIG pulsesuntil cycle complete(why?) • Tpulse  1.1RAC

4. 555 in astable mode • Q=1 (Q’=0), C charges through RA+RB • When VC>2/3VCC, the R input ofthe internal FF becomes 1 • The FF flips, Q=0 (Q’=1),transistor Q1 opens,C discharges through RB • When VC<1/3VCC, the S inputof the internal FF becomes 1 • The FF flops, Q=1 (Q’=0),transistor Q1 closes,C charges through RA+RB • tH  0.69(RA+RB)C • tL  0.69RBC • T  0.693(RA+2RB)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?