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

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.

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.

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

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.

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?

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

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

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? ? ?

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.

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

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.

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

NAND SR Latch S R Q Q’ Mode 1 Invalid Set Reset Hold 1 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.

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

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.

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

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

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

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

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.

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.

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

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

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.

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

END ©Paul R. Godin prgodin°@ gmail.com