208 Chapter 11 Summary The toggling flip-flop is the basis of all counter circuits, which use the clock as an input and the Q outputs as the outputs. In such a circuit, the Q outputs provide a binary number which represents the number of complete clock pulses since the counter was reset. Counter Uses Apart from their obvious applications to clocks and watches, counters are used extensively in electronics, and particularly in modern measuring instruments. ICs exist to provide for measurement of all the common quantities such as direct current (DC) voltage, signal peak and root mean square (RMS) amplitude, and frequency, using counting methods. Most of the modern ICs of this type provide for digital representation, so that the outputs are in a form suitable for feeding to digital displays. The simplest application of counters to instruments is for frequency measurement. Frequency Meter A frequency meter makes use of a high-stability oscillator, normally crystal controlled, to provide clock pulses. In its simplest form, the block diagram is as shown in Figure 11.5, with the unknown frequency used to open a gate for the clock pulses. The number of clock pulses passing through the gate in the time of one cycle of the input will provide a measure of the input frequency as a fraction of the clock rate. For example, if the clock rate is 10 MHz and 25 pulses of the unknown frequency pass the gate, then the input frequency is 10/25 MHz, which is 400 kHz. A counter circuit can find the number and display it in terms of the clock frequency to give a direct reading of the input frequency. In this simple form, the frequency meter could not cope with input frequencies that are greater than the clock rate, nor with input frequencies that would be irregular sub-multiples. For example, it cannot cope with 3.7 gated pulses in the time of a clock cycle. Both of these problems can be solved by more advanced designs. The problem of high frequencies can be Figure 11.5: Simplified block diagram for a frequency meter