# Bistable and D-Type Flip-Flop Circuits

### Overview

Bistables and D-Type flip flips are two very similar circuits The state of the digital output (either ON or OFF) not only depends on the state of the inputs but also on the state of the inputs in the past. These circuit are a type of memory circuit remembering what happened to the inputs some time previously. The output responds to a change in the inputs and then maintain the same state even if the inputs subsequently change. The output can be turned ON with one input and turned OFF with a different input, such as in an alarm system. These circuits are called synchronous logic circuits (logic circuits that depend on time) rather than combinational logic circuits that depend only on the state of the inputs at that particular moment.

### Learning Objectives

• Draw the circuit diagram and timing diagram for an Set-Reset (S-R) bistable circuit
• Appreciate that a bistable can be used as a simple memory and in alarm circuits
• Draw the circuit diagram and timing diagram for a (rising edge triggered) D-Type flip-flop circuit
• Appreciate how a D-Type flip-flop can be used as a simple memory circuit
• Understand how feedback is used with a D-Type flip-flop to make toggle circuits and a divide by two counter circuit

### Lesson Content

Reading: Read the first three sections of the bistable website page - Introduction, Bistable Basics and 4043 Bistable IC. Appreciate the names of the two inputs and what each one does. Draw the circuit diagram of a bistable circuit with two inputs.

Video (3 min): Watch the short video about what a bistable circuit does. The oscilloscope trace shows the timing diagram in real time.

Reading: Read the remaining sections of the bistable website page starting with the 4013 IC and then considering bistable circuits made from logic gates. Foe each of the latter sections there is a short Interactive White Board (IWB) video to watch.

Video (4 min): IWB video explaining the NOT gate bistable.

Video (7 min): IWB video explaining the NOR gate bistable.

Video (6 min): IWB video explaining the NAND gate bistable.

Reading: Read the first fout sections of the D-Type flip-flop website page covering the introduction, operation, timing diagram and the 4013 IC. Appreciate the circuit has two inputs called Ck and D as well as the S and R inputs of the bistable circuit. Understand how the Ck and D inputs have to be used together whereas the S and R inputs can each be used independently.

Video (4 min): A video explaining the operation of the D-Type flip-flop using the Ck and D inputs to change the state of an output, Q.

Reading: Read the remaining sections of the D-Type flip-flop website page which look at different applications of the D-Type flip-flop. Pay particular attention to the idea of feedback in the toggle circuit and the divide by two counter.

Video (4 min): Watch the video explaining the divide by two counter circuit and the important idea of feedback.

Exercises: Complete the questions and use the answers to check your understanding.

### Lesson Review

Review your learning by working through the presentations or notes which summarise the website content.

Presentation: Powerpoint download. Bistable circuits, useful ICs and timing diagram.

Notes: PDF download. Website content as a pdf document.

Presentation: Powerpoint download. D-Type flip-flop circuits, timing diagrams and example applications including the divide by two counter.

Notes: PDF download. Website content as a pdf document.

### Self Assessment

Complete either the questions (pdf download) OR the on-line quiz. They are the same questions.

Questions: PDF download. Questions about Bistable circuits and D-Type flip-flop circuits.

Quiz: Online questions about Bistable circuits and D-Type flip-flop circuits.

### Self Evaluation

I can:

• Describe the function of a Set-Reset (S-R) Bistable circuit
• State the relationship between the outputs Q and Q
• Interpret and/or draw the timing diagram for a bistable circuit
• Explain how bistable circuits can be used in alarm systems and as simple memory circuits
• Appreciate the difference between NOR logic and NAND logic when applied to the inputs of bistable circuits
• Describe the function of a D-Type flip-flop circuit using Clock (Ck) and Data (D) to change the state of the output
• Appreciate the a D-Type flip-flop circuit may also have Set (S) and Reset (R) inputs and that these take priority over the Ck and D inputs
• Understand the concept of a "rising edge" for the Clock input
• Interpret timing diagram for a D-Type flip-flop circuit
• Draw timing diagram for a given D-Type flip-flop circuit
• Design circuits using D-Typle flip-flops and bistables from a written description or timing diagram
• Understand how feedback can be used to make toggle circuits
• Appreciate that switch bounce may be an issue when using a push button as the Clock input but this does not affect the Date, Set and Reset inputs
• Appreciate that feedback can be used to make a divide by 2 circuit (which is the same as a toggle circuit)
• Draw the timing diagram for a divide by 2 circuit using feedback and appreciate that Q must necessarily change state after Q
• Appreciate how several D-Type flip-flops with a common clock can be used to make a data latch