1. Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flop
Flip-Flops and Latches
Digital Electronics
© 2014 Project Lead The Way, Inc.
Project Lead The Way, Inc.
Copyright 2009

2. Flip-FLops and Latches
Flip-Flops & Latches
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
This presentation will
Review sequential logic and the flip-flop.
Introduce the D flip-flop and provide an excitation table and a sample timing analysis.
Introduce the J/K flip-flop and provide an excitation table and a sample timing analysis.
Review flip-flop clock parameters.
Introduce the transparent D-latch.
Discuss flip-flop asynchronous inputs.
Introductory Slide / Overview of Presentation
Project Lead The Way, Inc.
Copyright 2009

3. Flip-Flops & Latches
The Flip Flop is a logic device that is capable of storing a logic level and allowing this stored value to change only at a specific time.
The Flip Flop is the fundamental building block for all sequential logic designs.

4. Sequential Logic & The Flip-Flop
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
Combinational
Logic Gates
Inputs
Outputs
Memory Elements
(Flip-Flops)
Clock
Sequential logic can have one or more inputs, and one or more outputs.
However, the outputs are a function of both the present value of the inputs
and also the previous output values. Thus, sequential logic requires
memory to store these previous outputs values.
Project Lead The Way, Inc.
Copyright 2009

5. D Flip-Flop: Excitation Table
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
CLK D Q D
CLK  1
 : Rising Edge of Clock
Schematic symbol and excitation table for the D flip-flop.
Project Lead The Way, Inc.
Copyright 2009

6. Flip-FLops and Latches
Clock Edges
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
Rising Edge
Positive Edge Transition 1
Negative Edge Transition
Falling Edge
Project Lead The Way, Inc.
Copyright 2009

7. POS & NEG Edge Triggered D
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
Positive Edge Trigger
CLK D Q D
CLK  1
 : Rising Edge of Clock
Schematic symbol and excitation table for the positive edge triggered and negative edge triggered D flip-flops
Negative Edge Trigger
CLK D Q D
CLK  1
 : Falling Edge of Clock
Project Lead The Way, Inc.
Copyright 2009

8. D Flip-Flop: Example Timing
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
Q=D=1
Q=D=0
Q=D=0
No Change
Q=D=1
Q=D=1
No Change
Q=D=0
Q=D=0
No Change Q D
CLK
Timing diagram example for a D flip-flop.
Project Lead The Way, Inc.
Copyright 2009

9. J/K Flip-Flop: Excitation Table
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops K J Q
CLK J K
CLK 
No Change 1
Clear
Set
Toggle
 : Rising Edge of Clock
Schematic symbol and excitation table for the J/K flip-flop.
Project Lead The Way, Inc.
Copyright 2009

10. J/K Flip-Flop: Example Timing
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops NO
CHANGE NO
CHANGE
SET
TOGGLE
TOGGLE
CLEAR
SET Q J K
CLK
Timing diagram example for a J/K flip-flop.
Project Lead The Way, Inc.
Copyright 2009

11. POS & NEG Edge Triggered J/K
Flip-FLops and Latches
POS & NEG Edge Triggered J/K
Digital Electronics TM
3.1 Introduction to Flip-Flops
Positive Edge Trigger K J Q
CLK J K
CLK  1
 : Rising Edge of Clock
Negative Edge Trigger
Schematic symbol and excitation table for the positive edge triggered and negative edge triggered J/K flip-flops K J Q
CLK J K
CLK  1
 : Falling Edge of Clock
Project Lead The Way, Inc.
Copyright 2009

12. Flip-FLops and Latches
Flip-Flop Timing
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
Data Input
(D,J, or K) 1 tS
Setup Time tH
Hold Time
Positive
Edge
Clock
Definition of the Setup & Hold Time timing parameters for a flip-flop.
Setup Time (tS): The time interval before the active transition of the clock signal during which the data input (D, J, or K) must be maintained.
Hold Time (tH): The time interval after the active transition of the clock signal during which the data input (D, J, or K) must be maintained.
Project Lead The Way, Inc.
Copyright 2009

13. Flip-FLops and Latches
Asynchronous Inputs
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
Asynchronous inputs (Preset & Clear) are used to override the clock/data inputs and force the outputs to a predefined state.
The Preset (PR) input forces the output to:
The Clear (CLR) input forces the output to:
CLK D Q PR
CLR
Definition for the PR (preset) and CLR (clear) Asynchronous input for a D flip-flop. PR
PRESET
CLR
CLEAR
CLK
CLOCK D
DATA 1  X
Asynchronous Preset
Asynchronous Clear
ILLEGAL CONDITION
Project Lead The Way, Inc.
Copyright 2009

14. D Flip-Flop: PR & CLR Timing
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
Q=D=1
Clocked
Q=D=0
Clocked
Q=D=0
Clocked
Q=D=1
Clocked
Q=D=1
Clocked
Q=D=0
Clocked Q PR
CLR D
CLK
Q=1
Preset
Q=1
Preset
Q=0
Clear
Time diagram showing the effects of the synchronous inputs (D & CLK) and asynchronous inputs (PR & CLR).
Project Lead The Way, Inc.
Copyright 2009

15. Flip-FLops and Latches
Transparent D-Latch
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops EN D Q EN D X 1
Schematic symbol and excitation table for the D latch.
EN: Enable
Project Lead The Way, Inc.
Copyright 2009

16. Transparent D-Latch: Example Timing
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
“Latched”
Q=0
“Transparent”
Q=D
“Latched”
Q=1
“Transparent”
Q=D
“Latched”
Q=0
“Transparent”
Q=D Q D EN
Time diagram example for a transparent D-latch.
Project Lead The Way, Inc.
Copyright 2009

17. Flip-FLops and Latches
Flip-Flop Vs. Latch
Digital Electronics TM
3.1 Introduction to Flip-Flops
The primary difference between a D flip-flop and D latch is the EN/CLOCK input.
The flip-flop’s CLOCK input is edge sensitive, meaning the flip-flop’s output changes on the edge (rising or falling) of the CLOCK input.
The latch’s EN input is level sensitive, meaning the latch’s output changes on the level (high or low) of the EN input.
This slide details the primary difference between the often confused D flip-flop and D latch.
Project Lead The Way, Inc.
Copyright 2009

18. Flip-FLops and Latches
Flip-Flops & Latches
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
74LS74
Dual Positive-Edge-Triggered D Flip-Flops with Preset, Clear, and Complementary Outputs
74LS76
Dual Negative-Edge-Triggered J-K Flip-Flops with Preset, Clear, and Complementary Outputs
Summary of the two flip-flops and one latch that we will be using in this course.
74LS75
Quad Latch
Project Lead The Way, Inc.
Copyright 2009

19. Flip-FLops and Latches
74LS74: D Flip-Flop
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
Datasheet excerpts for a 74LS74 D flip-flop.
Project Lead The Way, Inc.
Copyright 2009

20. Flip-FLops and Latches
74LS76: J/K Flip-Flop
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
Datasheet excerpts for a 74LS76 J/K flip-flop.
Project Lead The Way, Inc.
Copyright 2009

21. Flip-FLops and Latches
74LS75: D Latch
Flip-FLops and Latches
Digital Electronics TM
3.1 Introduction to Flip-Flops
Datasheet excerpts for a 74LS75 D latch.
Project Lead The Way, Inc.
Copyright 2009