All flip flops need some combination of inputs which programs their state, and some combination of inputs lets them maintain their state. to be the inputs, as well as the triggers for the state change. Without a clock input, it will either ignore its D input (useless!), or simply copy the input at all times (not a flip-flop!) An RS flip-flop doesn't have a clock, but it uses two inputs to control the state which allows the inputs to be 'self clocking': i.e. We allow a generous amount of time for the inputs to settle and then we indicate to the circuit to accept the values.Ĭlocking is also inherently part of the semantics of some kinds of flip flops.Ī D flip flop cannot be defined without a clock input. If we do not want the other circuits which depend on the output to see the chaos, we make the circuit clocked. If the output is instantaneously produced from the inputs, then it will be chaotic until the inputs stabilize.

Sometimes a circuit has many inputs, which do not stabilize at the same time. We have an orderly, predictable behavior which steps through the binary numbers without any glitch.Ĭlocking behaviors are useful in other situations too. Now, whenever the clock edge arrives, the register will accept the new value which is one plus its previous value.

This value is then simply fed back to the register. The output of the register is put through a combinatorial logic function which adds 1 (a four bit adder) to produce the incremented value. We can do this by using a 4 bit register (which is a bank of four D flip-flops). This clocking allows us to build computers, which are state machines: they have a current state, and calculate their next state based on the current state and some inputs.įor example, suppose we want to build a machine which 'computes' an incrementing 4 bit count from 0000 to 1111, and then wraps around to 0000 and keeps going. If a flip-flop's output is used to calculate its input, it behooves us to have orderly behavior: to prevent the flip-flop's state from changing until the output (and hence the input) is stable. One reason we clock flip flops so that there isn't any chaos when the outputs of flip flops are fed through some logic functions and back to their own inputs.

.flipclock is not a function