SPICE

D-Flip-Flop

POSITIVE EDGE TRIGGERED FLIP FLOP

From the introduction it is clear that for a positive edge triggered flip flop the changes in output occurs at the transition level. This is done by configuring two D latches in master slave configuration.A master slave D flip-flop is created by connecting two gated D latches in series, and inverting the clock input to one of them. It is called master slave because the second latch in the series only changes in response to a change in the first (master) latch. To understand the transistor level design of positive edge triggered flip flop study the two diagrams below

Positive edge triggered flip flop when clock=0

As evident from the figure when clk is 0 the input D passes through the first level of pass transistor logic and held there because the second level does not pass on the value of D

Positive edge triggered flip flop when clock=1

When the clock input becomes 1, D(at that instant) is transferred to the output. Thereafter output Q does not change when D changes because D is not passed through the first level of pass transistor logic (as seen in the diagram). Now when the clock changes back to 0, Q still remains unaffected by the changes in D because it is now hindered by the second level of pass transistor. Thus we observe that Q remains unchanged for the entire clock cycle and changes only at the positive edge. Hence the above transistor level diagram implements positive edge triggered flip flop.

APPLICATION AND ADVANTAGES OF D- FLIP FLOP

D flip flop can be considered as a basic memory cell because it stores the value on the data line with the advantage of the output being synchronised to a clock. D flip flops form the basis of shift registers that are used in many electronic devices. Many logic synthesis tools use only D flip flop or D latch. FPGA contains edge triggered flip flops. D flip flops are also used in finite state machines.

What to build in the practice Section

Given the introduction of a positive D-Flip Flop, use that as a reference to build a negative edge D-Flip-Flop in the practice section.