1. Flip Flops

2. Objectives SR latch SR Latch with enable (SR Flip-Flop) D Flip-Flop
JK Flip-Flop
T Flip-Flop

3. Flip Flop
It is a type of logic circuit whose output depends not only on the present input signals but on the sequence of past inputs.
A basic sequential circuit is a flip-flop
Flip-flop has two stable states of output values
SR Latch

4. 2018/4/26
Flip-Flops
Latches are “transparent” (= any change on the inputs is seen at the outputs immediately).
This causes synchronization problems.
Solution: use latches to create flip-flops that can respond (update) only on specific times (instead of any time).
Types: RS flip-flop ,D flip-flop etc.

5. SR Latch (NAND version)
2018/4/26
SR Latch (NAND version)
S’ R’ Q Q’ S’ 1 Q
0 0
0 1
1 0
1 1
1 0 Set Q’ 1 R’
X Y NAND

6. SR Latch (NAND version)
2018/4/26
SR Latch (NAND version)
S’ R’ Q Q’ 1 S’ 1 Q
0 0
0 1
1 0
1 1
1 0 Set Q’ 1 R’
1 0 Hold
X Y NAND

7. SR Latch (NAND version)
2018/4/26
SR Latch (NAND version)
S’ R’ Q Q’ 1 S’ Q
0 0
0 1
1 0
1 1
1 0 Set
0 1 Reset 1 Q’ R’
1 0 Hold
X Y NAND

8. SR Latch (NAND version)
2018/4/26
SR Latch (NAND version)
S’ R’ Q Q’ 1 S’ Q
0 0
0 1
1 0
1 1
1 0 Set
0 1 Reset 1 Q’ 1 R’
1 0 Hold
0 1 Hold
X Y NAND

9. SR Latch (NAND version)
2018/4/26
SR Latch (NAND version)
S’ R’ Q Q’ S’ 1 Q
1 1 Disallowed
0 0
0 1
1 0
1 1
1 0 Set
0 1 Reset 1 Q’ R’
1 0 Hold
0 1 Hold
X Y NAND

10. SR Latch with Enable (SR Flip-Flop)
The S and R inputs only effect the output states when the enable input C is high.
This controls when the latch responds to its inputs.
The latch holds (stores) its value while the enable input is low — latches it!
Any changes in the inputs during the time when enable is high will affect the output immediately: the circuit is said to be transparent.
This circuit still has a major problem: the stored value is indeterminate if S = R = 1 when the clock goes low

11. D Flip-Flop
The problem with S = R = 1 can be avoided using a common input D.

12. D Flip-Flop Qn+1 C D Q Q D Qn Qn+1 = D description Clear (reset) 1 Set
Clear
(reset) 1
Set
input at clock
output before clock
output after clock
Qn+1 = D
A D flip-flop simply stores the value on its D input at the clock transition. The previous value stored, Qn, has no effect, unlike other flip-flops.
It therefore acts as a simple memory or ‘latch’.
The most widely used flip-flops: simple to build and design with.
A register comprises several D flip-flops, one for each bit to be stored.

13. JK Flip-Flop

14. JK Flip-Flop Qn+1 J Q K Q J K Qn Qn+1 = J ◊ Qn + K ◊ Qn description
hold 1
clear (reset)
set
toggle
J Q K Q
Qn+1 = J ◊ Qn + K ◊ Qn
The excitation table for a JK flip-flop is similar to SR flip-flop but doesn’t have the problem of S = R = 1. It can perform all the operations of the simpler types of flip-flop. However, the design of the circuit internal to the flip-flop makes it more expensive to manufacture than a number of other flip-flops so JK flip-flops are now rarely used.

15. T-flip-flop

16. Toggle (T) Flip-Flop Qn+1 T Q Q T Qn Qn+1 = T ≈ Qn = T ◊ Qn + T ◊ Qn
description
hold 1
toggle
Qn+1 = T ≈ Qn = T ◊ Qn + T ◊ Qn
The output depends on the previous value stored, Qn.
This type of flip-flop is rarely bought as a ‘dedicated’ device — you can easily make a T from a D flip-flop.

17. Thank you