11
Counters and Registers
Counters and registers belong to the category of MSI sequential logic circuits. They have similar
architecture, as both counters and registers comprise a cascaded arrangement of more than one flipflop with or without combinational logic devices. Both constitute very important building blocks of
sequential logic, and different types of counter and register available in integrated circuit (IC) form are
used in a wide range of digital systems. While counters are mainly used in counting applications, where
they either measure the time interval between two unknown time instants or measure the frequency of
a given signal, registers are primarily used for the temporary storage of data present at the output of a
digital circuit before they are fed to another digital circuit. We are all familiar with the role of different
types of register used inside a microprocessor, and also their use in microprocessor-based applications.
Because of the very nature of operation of registers, they form the basis of a very important class of
counters called shift counters. In this chapter, we will discuss different types of counter and register as
regards their operational basics, design methodology and application-relevant aspects. Design aspects
have been adequately illustrated with the help of a large number of solved examples. A comprehensive
functional index of a large number of integrated circuit counters and registers is given towards the end
of the chapter.
11.1 Ripple (Asynchronous) Counter
A ripple counter is a cascaded arrangement of flip-flops where the output of one flip-flop drives the
clock input of the following flip-flop. The number of flip-flops in the cascaded arrangement depends
upon the number of different logic states that it goes through before it repeats the sequence, a parameter
known as the modulus of the counter.
In a ripple counter, also called an asynchronous counter or a serial counter, the clock input is applied
only to the first flip-flop, also called the input flip-flop, in the cascaded arrangement. The clock input
to any subsequent flip-flop comes from the output of its immediately preceding flip-flop. For instance,
the output of the first flip-flop acts as the clock input to the second flip-flop, the output of the second
flip-flop feeds the clock input of the third flip-flop and so on. In general, in an arrangement of n
Digital Electronics: Principles, Devices and Applications Anil K. Maini
© 2007 John Wiley & Sons, Ltd. ISBN: 978-0-470-03214-5
Counters and Registers
Counters and registers belong to the category of MSI sequential logic circuits. They have similar
architecture, as both counters and registers comprise a cascaded arrangement of more than one flipflop with or without combinational logic devices. Both constitute very important building blocks of
sequential logic, and different types of counter and register available in integrated circuit (IC) form are
used in a wide range of digital systems. While counters are mainly used in counting applications, where
they either measure the time interval between two unknown time instants or measure the frequency of
a given signal, registers are primarily used for the temporary storage of data present at the output of a
digital circuit before they are fed to another digital circuit. We are all familiar with the role of different
types of register used inside a microprocessor, and also their use in microprocessor-based applications.
Because of the very nature of operation of registers, they form the basis of a very important class of
counters called shift counters. In this chapter, we will discuss different types of counter and register as
regards their operational basics, design methodology and application-relevant aspects. Design aspects
have been adequately illustrated with the help of a large number of solved examples. A comprehensive
functional index of a large number of integrated circuit counters and registers is given towards the end
of the chapter.
11.1 Ripple (Asynchronous) Counter
A ripple counter is a cascaded arrangement of flip-flops where the output of one flip-flop drives the
clock input of the following flip-flop. The number of flip-flops in the cascaded arrangement depends
upon the number of different logic states that it goes through before it repeats the sequence, a parameter
known as the modulus of the counter.
In a ripple counter, also called an asynchronous counter or a serial counter, the clock input is applied
only to the first flip-flop, also called the input flip-flop, in the cascaded arrangement. The clock input
to any subsequent flip-flop comes from the output of its immediately preceding flip-flop. For instance,
the output of the first flip-flop acts as the clock input to the second flip-flop, the output of the second
flip-flop feeds the clock input of the third flip-flop and so on. In general, in an arrangement of n
Digital Electronics: Principles, Devices and Applications Anil K. Maini
© 2007 John Wiley & Sons, Ltd. ISBN: 978-0-470-03214-5