1. Software Developers View of Hardware Understanding Flip-Flops

2. What is a Flip-Flop?

• A flip-flop is a bistable device.

• This means that output from the device can be one of two possible states and will remain that way even after input signals are removed.

• A flip-flop will only change state when commanded.

• Because of this they form the basis of memory.

3. Components

• A flip-flop has three (3) basic components:

• • Latch

• • Clock

• • Trigger

4. Latches

• The most basic of all latches is the RS latch.

• There are TWO types of RS latches, they are:

NOR Latch NAND Latch

5. Latches

• The basic understanding behind a latch is that if 1 is sent as the SET input then the output (Q) will be 1.

• Likewise, if 1 is sent to the RESET input then the output (Q) will be 0.

• The most important part is FEEDBACK which enables the latch to carry out the task of enabling memory.

6. NOR Latch

• Firstly, lets look at the RESET condition. (Send 1 as RESET input)

1 0

7. Look at the Truth!!!!!! 0 1 1 0 0 1 0 1 0 1 0 0 X B A

8. Look at the Truth!!!!!! 0 1 1 0 0 1 0 1 0 1 0 0 X B A

9. NOR Latch

• So therefore, if an at least one input is a 1 then the only possible output can be 0.

1 0

10. NOR Latch

• So therefore, if an at least one input is a 1 then the only possible output can be 0.

1 0 0 0 1 1

11. NOR Latch

• Next, lets look at the SET condition. (Send 1 as SET input)

0 1

12. NOR Latch

• Next, lets look at the SET condition. (Send 1 as SET input)

0 1 0 0 1 1

13. NOR Latch

• Next, lets look at the HOLD condition. (Will hold what ever was the previous output)

0 0 0 1

14. NOR Latch

• Next, lets look at the HOLD condition. (Will hold what ever was the previous output)

0 0 0 1 0 1

15. NOR Latches

• This is where the see saw effect comes into play:

16. NOR Latch

• Finally, for a NOR latch when 1 and 1 are both entered this violates logic rules because Q and NOT Q cannot be the same.

• It is referred to as being illegal.

17. NOR Latch Truth Table

18. NAND Latch

• Firstly, lets look at the RESET condition. (Send 1 as RESET input)

0 1

19. Look at the Truth!!!!!! 0 1 1 1 0 1 1 1 0 1 0 0 X B A

20. Look at the Truth!!!!!! 0 1 1 1 0 1 1 1 0 1 0 0 X B A

21. NAND Latch

• Firstly, lets look at the RESET condition. (Send 1 as RESET input)

0 1 1 1 0 0

22. NAND Latch

• Next, lets look at the SET condition. (Send 1 as SET input)

1 0 1 1 0 0

23. NAND Latch

• Next, lets look at the if 1 is sent to both inputs.

0 1

24. NAND Latch

• A point to remember is that a NAND flip flop works oppositely to a NOR flip flop so an input of (0 0) is illegal. However, an input of (1 1) cause the gate to remember the previous input.

25. NAND Latch

• Next, lets look at the if 1 is sent to both inputs.

1 1 What you will notice is that it depends on the see saw.

26. NAND Latch

• Next, lets look at the if 1 is sent to both inputs.

1 1 What you will notice is that it depends on the see saw.

27. NAND Latch

• Next, lets look at the if 1 is sent to both inputs.

1 1 What you will notice is that it depends on the see saw. 1 0

28. NAND Latch

• Complete the following truth table depending on the inputs shown.

1 0 1 1 Q Q D C B A

29. Clocked RS Latch

• The logic behind this logic gate is the fact that there is another input called ENABLED.

• This acts like a gate or a switch and when set to 1 the circuit will respond as usual. However, when it is set to zero the circuit will not respond.

30. Clocked RS Latch

31. Clocked RS Latch