changes state after every n input pulses, the output frequency is 1/n of the input fre-quency. Counters are built up by interconnecting many flip-flops.A special type of counter can be instructed to divide by any desired positive inte-ger n. This is a programmable divider. It forms the heart of a digital frequency syn-thesizer, an oscillator whose frequency can be digitally set. You learned about thiscircuit back in chapter 25.The registerA register is a combination of flip-flops that can delay digital signals, or store them fora short time.Shift registersIn a shift register, the flip-flops pass signal bits along from one to the next each time atriggering pulse is received. The most common use for a shift register is to produce atimed delay in digital data.Any input signal to a shift register will eventually arrive at the output. The outputbits occur in the same order as the input bits, except later. The length of the delay canbe set by changing the clock (triggering) pulse frequency. Higher clock frequencies re-sult in less delay, and lower frequencies result in more delay.Storage registersIn a storage register, a byte, word, or group of words is held for awhile for later use.This allows the circuit to work as a small-capacity, short-term memory. Storage regis-ters are common in digital computers.The register567Table 30-9. Truth tablefor a J-K Hip-flop.JKQQ00QQ0110100111QQ30-7Schematic symbol for aJ-K flip-flop.