Timing circuits Monostable multivibrators oneshots Digital storage circuit

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

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

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>2/3 VCC – Ignores new TRIG pulses until cycle complete (why? ) – Tpulse 1. 1 RAC

555 in astable mode – Q=1 (Q’=0), C charges through RA+RB – When VC>2/3 VCC, the R input of the internal FF becomes 1 – The FF flips, Q=0 (Q’=1), transistor Q 1 opens, C discharges through RB – When VC<1/3 VCC, the S input of the internal FF becomes 1 – The FF flops, Q=1 (Q’=0), transistor Q 1 closes, C charges through RA+RB – t. H 0. 69(RA+RB)C – t. L 0. 69 RBC – T 0. 693(RA+2 RB)C

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

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

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