3. Latches
Multivibrator
Digital Circuit
One bit of information
Temporary Storage Device
Bistable
Similar to Flip-Flop
outputs are constantly
Output changes by inputs
4. The SR Latch
Asynchronous device
Works independently of control signals
Two inputs S‘ and R' for set and reset respectively
Two outputs Q and Q‘.
A set state when Q = 1
a reset state when Q = 0
5.
6. The Gated S-R Latch
Enable input(EN)
sensitive to its inputs all the time
disable the inputs
When E =1, the circuit behaves like the normal NAND
implementation of the SR latch except that the S and R inputs are
active high rather than low.
When E = 0, the latch remains in its previous state regardless of the
S and R inputs.
7.
8. D Latch
D for "data“
Transparent Latch
Simple extension of the gated SR latch
Removes the possibility of invalid input states
The D latch outputs the D input whenever the Enable line is high
10. Gated D Latch
When the E input is asserted (E = 1), the Q output follows the D
input
the latch is said to be “open” and the path from the input D to the
output Q is “transparent”.
When E is de-asserted (E = 0), the latch is disabled or “closed”, and
the Q output retains its last value independent of the D input
11. The Edge-Triggered Flip-Flops
Synchronous Bistable Devices
Bistable Multivibrators
Clock pulse
Clock (CLK)
Not available in IC Form
12.
13. The Edge-Triggered S-R Flip-Flop
sequential circuits
two gates
cross-coupling
two active low inputs marked S and R
14. The Edge Triggered D Flip-Flop
The Q output always takes on the state of the D input at the
moment of a rising clock edge
The D flip-flop can be interpreted as a primitive memory cell, zero-
order hold, or delay line
These flip flops are very useful, as they form the basis for shift
registers, which are an essential part of many electronic devices
15. Clock D Q Qprev
Rising edge 0 0 X
Rising edge 1 1 X
Non-Rising X Qprev
16. The Edge Triggered J-K Flip-Flop
The JK flip-flop augments the behavior of the SR flip-flop (J=Set,
K=Reset) by interpreting the S = R = 1 condition as a "flip" or
toggle command
The characteristic equation of the JK flip-flop is:
17. JK Flip Flop operation
Characteristic table Excitation table
J K Qnext
Comme
nt
Q Qnext J K
Comme
nt
0 0
hold
state
0 0 0 X
No
change
0 1 reset 0 1 1 X Set
1 0 set 1 0 X 1 Reset
1 1 toggle 1 1 X 0
No
change