The counter

Chapter The counter

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • SynchronousAn R-S flip-flop output changes state as soon as the inputs change. If the inputs changeat irregular intervals, so will the outputs. For this reason, the aforementioned circuit issometimes called an asynchronous flip-flop.A synchronous flip-flop changes state only at certain times. The change-of-statetimes are determined by a circuit called a clock. The clock puts out a continuous trainof pulses at regular intervals.There are several different ways in which a synchronous flip flop can be triggered,or made to change state.In static triggering, the outputs can change state only when the clock signal is ei-ther high or low. This type of circuit is sometimes called a gated flip-flop.In positive-edge triggering, the outputs change state at the instant the clock sig-nal goes from low to high, that is, while the clock pulse is positive-going. The term “edgetriggering” derives from the fact that the abrupt rise or fall of a pulse looks like the edgeof a cliff (Fig. 30-6). In negative-edge triggering, the outputs change state at the in-stant the clock signal goes from high to low, or when the pulse is negative-going.566 Basic digital principles30-6Pulse edges are eitherpositive-going ornegative-going.Master/slaveWhen two flip-flops are triggered from the same clock, or if one of the outputs is usedas an input, the input and output signals can sometimes get confused. A master slave(M/S) flip-flop overcomes this bugaboo by storing inputs before allowing the outputs tochange state.An M/S flip-flop consists of two R-S flip-flops in series. The first flip-flop is called themaster, and the second is called the slave. The master flip-flop functions when theclock output is high, and the slave acts during the next low portion of the clock output.This time delay prevents confusion between the input and output of the circuit.J-KThe J-K flip-flop works just like an R-S circuit, except that it has a predictable outputwhen the inputs are both 1. Table 30-9 shows the input and output states for this typeof flip-flop. The output changes only when a triggering pulse is received.The symbol for a J-K flip-flop is shown in Fig. 30-7. It looks just like the R-S flip-flopsymbol, with the addition of a third (trigger) input, labeled T.The counterSome digital circuits can be programmed to change state every so-many clock pulses.Such a circuit is called a counter. It is also often called a divider, because if the output