Flip Flop Circuits (OCR A Level Computer Science): Revision Note
Exam code: H446
D Type Flip Flops
What is a D type flip flop?
In A Level Computer Science, a D type flip flop is a fundamental component in digital circuits and computer memory
Can be referred to as Positive Edge Triggered
Key features
Contains two stable states, making it a type of bistable circuit
Used to store the state of 1 bit of data
Changes state on the edge of the clock pulse
Components
Data input (D)
Clock input (CLK)
Two outputs: Q and NOT(Q)
D Type Flip Flip Components
Operation
On the rising edge of the clock pulse:
If D is high (1), Q goes high and NOT(Q) goes low
If D is low (0), Q goes low and NOT(Q) goes high
The state of Q holds or "remembers" its value until the next rising clock edge
Use cases
Forms the basis of most types of flip flops and latches
Used in shift registers, counters, and memory units
Helpful in edge-triggered devices, synchronous circuits, and data storage
Examiner Tips and Tricks
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
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