1. Flip-flops and Related Devices
• Multivibrators are electronic circuits used to
implement a variety of simple two-state systems
such as oscillators, timers and flip-flops.
• Multivibrators are applied in a variety of systems
where square waves or timed intervals are required
• Oscillators – systems that provide a repetitive
variation (in time) of some measure about a central
value. E.g. AC power, a swinging pendulum
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2. Flip-flops and Related Devices
• Multivibrator logic devices are of three
categories:
• Bistables – 2 stables states
• Monostables – 1 stable state
• Astables – no stable state
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3. Flip-flops and Related Devices
• Astable – the circuit is not stable in either state
– it continuously oscillates from one state to the
other.
• Monostable – one of the states is stable, but the
other is not – the circuit will flip into the unstable
state for a determined period, but will eventually
return to the stable state
• Bistable – the circuit will remain in either state
(stable or unstable) indefinitely. The circuit can
be flipped from one state to the other by an
external event or trigger.
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4. Bistables
• Bistables are of two types
– The latch
– The flip-flop
• Bistables have two stable states
– SET
– RESET
– They can retain either of these states indefinitely,
which makes them useful for storage purposes
• Latches and flip-flops differ in the way in which
they change from one state to another
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5. Latches
• A type of bistable storage device, can be in either of two
states (SET, RESET)
• The S-R (SET-RESET) latch
– An active-HIGH input S-R latch is formed with 2
cross-coupled NOR gates
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6. Latches
• An active-LOW input S-R latch
– Is formed with two cross-couples NAND gates
– The output of each gate is connected to an input of
the opposite gate
– Produces the regenerative feedback – characteristic
of all multivibrators
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7. Latches
• The S’-R’ latch (with negative OR equivalents
used for the NAND gates)
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8. Latches
• Assumption:
– Both inputs and the Q output are HIGH
– Q is connected as input to G2, and when R’ input it
HIGH, the output of G2 (Q’) is LOW; which is input to
G1, ensuring that its output (Q) is HIGH
– When Q is HIGH, the latch is in the SET state
– When R’ input is LOW and S’ is HIGH, the output of
G2 is HIGH
– Q’ output (HIGH) is coupled back to the input of G1,
since S’ is HIGH, the output of G1 is LOW
– When the Q output is LOW, the latch is in the
RESET state, and the latch remains there until a
LOW is applied to the S’ input
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9. Latches
• The outputs of a latch are always complements
of each other; when Q = HIGH, Q’ = LOW and
vice versa
• Invalid condition: occurs when LOWs are
applied to both S’ and R’ at the same time
• As long as the LOW levels are simultaneously
held in the inputs, both Q and Q’ outputs are
forced HIGH, violating the basic complementary
operation of the output
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10. Latches
• Modes of the S-R latch (SET, RESET, No
Change, Invalid Condition)
Inputs
Outputs
Comments
S’
R’
Q
Q’
1
1
NC
NC
No change
0
1
1
0
Latch SET
1
0
0
1
Latch RESET
0
0
1
1
Invalid condition
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11. Latches
• Logic symbols for the S-R latch
S
Q
S’
S
Q
R
Q’
R’
R
Q’
Active-HIGH input
S-R Latch
Active-LOW input
S’-R’ Latch
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12. Latches – Examples
• If the S’-R’ waveforms are applied to the inputs of the
latch, determine the waveform that will be observed on
the Q output. Assume that Q is initially LOW
S’
R’
Q
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13. Latches – Examples
• How to obtain the output (Q) waveform – use the
latch’s truth table to obtain Q for the given S’ and R’
inputs
• Initially, Q is 0
• S’R’
• 11 = No change
• 10 = Reset
• 01 = Set
R’
Q
0
1
1
1
1
1
1
0
0
0
1
1
1
1
1
1
0
0
1
1
0
1
0
0
1
1
0
0
1
1
1
1
1
0
1
1
1
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S’
1
1
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14. The Gated S-R latch
• Requires an ENABLE, EN, input
• The S and R inputs control the state to which the
latch will go when a HIGH level is applied to
the EN input
• The latch will not change until EN is HIGH, but
as long as it remains HIGH, the output is
controlled by the state of S and R inputs
• Invalid state occurs when both S and R are
simultaneously HIGH
– as opposed to the normal S-R where invalid state
occurs when both S and R inputs are LOW at the
same time
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15. The Gated S-R latch
• Logic diagram
• Logic symbol
S
Q
EN
Q’
R
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16. The Gated S-R latch
• Example: determine the Q output waveform if the inputs
shown are applied to the gated S-R latch that is initially
RESET (Q = 0)
S
R
EN
Q
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17. The Gated S-R latch
• Anytime S is HIGH and R is LOW, a HIGH
on the EN SETS the latch (Q = 1)
• Anytime S is LOW and R is HIGH, a HIGH
on the EN RESETS the latch (Q = 0)
– S R Q
– 1 1 Invalid
– 0 1 Reset (0)
– 1 0 Set (1)
– 0 0 No Change
R
EN
Q
1
0
1
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
1
0
1
1
0
0
1
0
0
1
0
0
0
1
0
0
0
1
0
1
1
1
0
1
1
0
1
1
0
0
1
1
0
1
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S
0
1
1
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18. The Gated S-R latch
• Exercise
– Determine the Q output of the gated S-R latch
if the S and R inputs are inverted. The latch is
initially RESET
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