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Flip-flops, counters and shift registers
Memory and sequencing in digital circuits.
Memory and sequencing in digital circuits.
A flip-flop is a circuit that remembers one bit. Gates alone have no memory: their output follows their inputs. A flip-flop feeds its output back so it holds its last state until told to change. Chain flip-flops together and you get counters, dividers and shift registers, the parts that give digital circuits memory and a sense of order.
The D (data) flip-flop is the workhorse. On each clock edge it copies D to Q, then holds Q until the next edge, however D wanders in between. The clock is what keeps a whole circuit in step: every flip-flop updates at the same instant, so signals can't race each other.
Two simpler cousins you will meet: the SR latch, which is set or reset directly with no clock, and the JK flip-flop, which toggles when both inputs are high. Tie a D flip-flop's inverted output back to its own D and it toggles on every clock edge.
A toggling flip-flop makes one output cycle for every two clock cycles, so it halves the frequency. Feed that output to the next flip-flop and halve again:
N flip-flops divide by 2ᴺ and, read together, count in binary from 0 to 2ᴺ − 1. A decade counter is arranged to count 0 to 9 then roll over, dividing by 10. Ten million hertz through seven decade counters is 1 Hz, which is how a crystal clock becomes a one-second tick. Dividers like this also sit inside synthesizers and in the prescalers that let a frequency counter handle VHF; see Frequency counters and signal generators.
A shift register is flip-flops in a line, each feeding the next. On every clock edge each bit moves one place. Clocking in eight bits one after another turns a serial stream into an 8-bit parallel word, or the reverse. Serial links, the control chips behind displays and band-switch outputs, and many radio data buses work this way: few wires, one bit per tick.