3-23
Clocks are designed to provide the most efficient operation of the equipment. During the design
phase, the frequency, pulse width, and the number of phases required is determined; and the clock circuit
is built to meet those requirements.
Most modern high-speed equipment uses crystal-controlled oscillators as the basis for their timing
networks. Crystals are stable even at extremely high frequencies.
Q33. What is a clock with regard to digital equipment?
Q34. What is the simplest type of clock circuit?
Q35. What is needed to use a monostable or one-shot multivibrator for a clock circuit?
Q36. What type of clock is used when more than one operation is to be completed during one clock
cycle?
COUNTERS
A counter is simply a device that counts. Counters may be used to count operations, quantities, or
periods of time. They may also be used for dividing frequencies, for addressing information in storage, or
for temporary storage.
Counters are a series of FFs wired together to perform the type of counting desired. They will count
up or down by ones, twos, or more.
The total number of counts or stable states a counter can indicate is called MODULUS. For instance,
the modulus of a four-stage counter would be 1610, since it is capable of indicating 00002 to 11112. The
term modulo is used to describe the count capability of counters; that is, modulo-16 for a four-stage
binary counter, modulo-11 for a decade counter, modulo-8 for a three-stage binary counter, and so forth.
Ripple Counters
Ripple counters are so named because the count is like a chain reaction that ripples through the
counter because of the time involved. This effect will become more evident with the explanation of the
following circuit.
Figure 3-23, view A, shows a basic four-stage, or modulo-16, ripple counter. The inputs and outputs
are shown in view B. The four J-K FFs are connected to perform a toggle function; which, you will recall,
divides the input by 2. The HIGHs on the J and K inputs enable the FFs to toggle. The inverters on the
clock inputs indicate that the FFs change state on the negative-going pulse.
