Flip Flop Circuits (OCR A Level Computer Science)

Exam Tip

You will not be asked to recall the logic gates that make up a D-type flip flop and they can be made from various different gates
They are often built using NAND gates which are AND gates that have their output inverted:

D Type Flip Flip Logic Gates

Worked Example

• Draw the output of the Flip Flop on the diagram below

D Type Flip Flop Question

• For a question like this remember that the output line Q that you are asked to draw wants to be the same as the input D BUT it can only change when the clock signal changes from low to high (i.e. at the dotted lines)

• At the start assume Q starts the same as D (low in this case)

• Then draw it along from left to right until you get to a vertical dotted line (in an exam you would draw these lines on to help you if they weren’t there

• At each dotted line, Q has the chance to change to whatever D is

  

D Type Flip Flop Working 1

• So at the first line, D is low so Q stays low

• At the second dotted line D has changed to high so now Q can become high

  

D Type Flip Flop Working 2

• Q now stays high until the next rising edge of the clock (the next dotted line) where it gets another chance to change, but here D is still high so Q stays high

D Type Flip Flop Working 3

• And you continue to do this until you reach the right hand side of the diagram

  

D Type Flip Flop Working 4