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 lt Vcc lt15V
Vthreshold gt 2/3Vcc resets the RS FF
Vtrigger lt 1/3Vcc sets the RS FF
3
Timing circuits

• The 555 integrated circuit timer in monostable
  mode
• Stable state Q0 (Q1), C is discharged (why?)
• Pulse U on TRIG sets Q1
• C charges through RA
• Until VCVTHRESHgt2/3VCC
• Ignores new TRIG pulsesuntil cycle
  complete(why?)
• Tpulse ? 1.1RAC

4
555 in astable mode

• Q1 (Q0), C charges through RARB
• When VCgt2/3VCC, the R input ofthe internal FF
  becomes 1
• The FF flips, Q0 (Q1),transistor Q1 opens,C
  discharges through RB
• When VClt1/3VCC, the S inputof the internal FF
  becomes 1
• The FF flops, Q1 (Q0),transistor Q1 closes,C
  charges through RARB
• tH ? 0.69(RARB)C
• tL ? 0.69RBC
• T ? 0.693(RA2RB)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?