1. Latches and Flip-Flops 1
©Paul Godin
Created September 2007
Last edit Aug 2013

2. Multivibrators
Multivibrators are a family of devices that produce output states.
There are 3 types of Multivibrators:
Bistable: Provide 2 stable states that can be held
Monostable: One-shot, timed output devices
Astable: Not stable. Oscillators that produce square waves
This presentation addresses the Bi-Stable Multivibrators.

3. Latches and Flip-Flops
Latches and Flip Flops are Bistable Multivibrators, meaning they provide 2 stable states.
These devices have the ability to hold, or “Latch” a state.

4. Examples of Latch circuits
Can you name some examples of devices that retain or latch onto an input?
Elevator call button
“soft” power switches, such as on a stereo or computer
Door lock, alarm system arming on a car
Memory devices (such as a flash drive)
Cap locks, number lock, insert/over key on a keyboard
Handsfree button on a telephone
Elevator buttons
Stereo amplifier or a computer power switch
Walk request button at a crosswalk
Computer memory
Car ignition
Electric car lock
Counting
Moving data
Arithmetic operations

5. Latch Example: The Industrial Push Button
Pushing “Start” causes the motor to start.
If the button is let go, the motor continues to run.
Pushing “Stop” causes the motor to stop.
If the button is let go, the motor is still in a stop state.
Start
Stop
The device holds the last input.

6. The Industrial Push Button
What happens if the motor is running and the start button is pushed again?
What happens if both the start and stop buttons are pushed at the same time?

7. Push Button Timing Diagram
Unknown State (both start and stop high)

8. Push Button Truth Table
Start Button
Stop Button
Motor
Low
Hold previous state
High
Stop
Start
Unknown

9. Review: NOR & NAND Gate
What logic input forces a change to the output of a NOR gate?
What logic input forces a change to the output of a NAND gate? ? ?

10. NOR Latch
In the NOR latch the output of a NOR gate is connected to an input of another in a feedback configuration.
The feedback configuration is needed for a hold state.

11. NOR SR Latch S R Q Q’ Mode Notes Hold 1 Reset Set Invalid
Hold
No active input 1
Reset
Q is ‘0’ on reset
Set
Q is ‘1’ on set
Invalid
Q must not = Q’
Boolean Violation

12. NOR SR Latch S R Q Q’ Mode Hold 1 Reset Set Invalid
Hold 1
Reset
Set
Invalid
Active high: a 1 input forces a 0 output state
Follow the active input state to determine the output of a latch.

13. NOR SR Latch Function 1- Apply a 1-0 input
2- The 1 forces the 0 output 1 1
3- The 0-0 input causes a 1 output
4- Active input is removed. Output is held. 1 1
The Latch is SET (Q=1)
The Latch is in a HOLD state

14. NAND SR Latch S R Q Q’ Mode 1 Invalid Set Reset Hold1
Invalid
Set
Reset
Hold
Active low; a 0 input forces a 1 output state.
Follow the active input state to determine the output of a latch.

15. NAND SR Latch Function 1- Apply a 0-1 input
2- The 0 forces the 1 output 1 1 1
3- The 1-1 input causes a 0 output
4- Active input is removed. Output is held. 1 1 1 1 1
The Latch is SET (Q=1)
The Latch is in a HOLD state

16. Active State
The NOR Latch is an active high device where the logic high forces the output.
Any 1 input to a NOR forces a 0 output.
The NAND Latch is an active low device where the logic low forces the output.
Any 0 input to a NAND forces a 1 output.

17. Logic diagram of an active-high SR Latch
Latches and Flip Flops S R Q
Inputs
Outputs
Logic diagram of an active-high SR Latch

18. Complete the truth tables for the latches
Exercise 1
Complete the truth tables for the latches S R Q S R Q S R Q Q’
Mode 1 S R Q Q’
Mode 1

19. Exercise 2: Timing Diagram
Complete the timing diagram S R Q S R Q Q

20. Exercise 3: Timing Diagram
Complete the timing diagram S R Q S R Q Q

21. Other Important Bistables
There are other important latches and flip-flops:
Gated SR Latch
Gated D-Latch
Edge-Triggered D Flip-Flop
J-K Flip Flop
These are addressed in the next presentation.

22. Important Consideration
SR Latches are the foundation of all Latches and Flip-Flops
Latches and Flip-Flops are a foundation to digital electronics.
It is CRITICAL that you take time to know the latches thoroughly. Check the course website for links to other resources.

23. Latch Application: Contact De-Bouncer
When switch contacts are pushed against each other they bounce.
Contact bounce can cause problems in digital circuits.
Animated

24. SR Latch De-bouncer Function
When the switch makes first contact, the latch is set (S=0, R=1).
Bouncing creates a hold state (S=1, R=1).
Vcc S R Q
Active low

25. SR Latch De-bouncer
The “bounce state”, where the switch is in a state of transition between the contacts, must create a hold input to the latch.

26. Exercise
Create a switch debouncer from an active-high SR Latch.

27. END
©Paul R. Godin
gmail.com