- Flip Flop Definition: A flip flop is a basic digital memory circuit that stores one bit of data and can be used in various applications by connecting multiple flip-flops.
- Registers and Shift Registers: Registers store multiple bits of data using flip-flops, while shift registers move data within or out of registers based on clock pulses.
- Counters: Flip-flops form counters that count events and can be designed in various ways to suit specific counting needs.
- Event Detection: Flip-flops can detect events by changing states and holding the new state until the event is cleared.
- Frequency Division: Flip-flops can divide input frequencies by toggling their output states, effectively reducing the frequency by half for each flip-flop stage.
Registers
Flip-flops are used wherever a circuit must store bits, count events, catch a pulse, align data to a clock or divide a frequency. A register is the simplest of those uses: each flip-flop stores one bit, so n flip-flops store n bits. Three D flip-flops hold B3, B2 and B1 and form a 3-bit buffer register.
Clock pulses can move the stored bits from stage to stage, or in and out of the register. That circuit is a shift register. The data path may be serial-in serial-out, serial-in parallel-out, parallel-in serial-out or parallel-in parallel-out. The shift may be left, right or both, as in Figure 2.
Counters
A counter is a chain of flip-flops (JK, D or T) wired to count events. One flip-flop has two states, 0 and 1, so it is a 1-bit modulo-2 (MOD-2) counter. To count eight states, connect three flip-flops (23 = 8) as in Figure 3. Clock wiring decides synchronous or asynchronous operation and rising-edge or falling-edge triggering. Other connections give an up-counter, a down-counter, an up/down counter, a ring counter or a johnson counter.
Event Detectors
An event detector changes state when a pulse arrives and holds that state until the event is cleared. A flip-flop does that because it keeps its output until its inputs change. A D flip-flop can, for example, remember that a light was switched on.
Data Synchronizers
A data synchronizer makes combinational outputs change together. Gate delays otherwise make those outputs switch at different times and can produce a wrong code. Clocked D flip-flops hold the bits and release them on the same clock edge.
The flip-flops keep the old bits until the rising clock edge. All outputs then update together.
Frequency Divider
Figure 6 is a rising-edge JK flip-flop with J and K tied high. It toggles on each rising clock edge (red lines). If the clock period is Tin, the output period Tout is 2 Tin, so fout = fin/2. One toggle stage halves the frequency. n stages in series divide by 2n, so fout = fin/2n.
Two more common uses:
- D flip-flop can be used to create delay-lines which are used in digital signal processing systems. This application arises readily due to the fact that the output at the synchronous D flip-flop is nothing but the input delayed by one-clock cycle. Thus by cascading n such flip-flops, output can be delayed by n clock cycles which in turn produces the required amount of delay.
- Generally the mechanical switches used to enter the values into the digital system are prone to bouncing problem where the switch-contacts vibrate while closing/opening the switch. This leads to the variation in the output voltage causing the logical inputs to alternate between 0 and 1. This results in unexpected system behavior which can be avoided by connecting a RS flip-flop between the switch and the digital circuit to act as a switch debouncer.
