Decoders and Multiplexers
Decoders and Multiplexers
A decoder is a circuit which has n inputs and 2n outputs, and
outputs 1 on the wire corresponding to the binary number represented
by the inputs. For example, a 2-4 decoder might be drawn like this:
and its truth table (again, really four truth tables, one for each
We can immediately see that
i.e. each of the d outputs corresponds to one of the four minterms.
The Decoder Circuit
The following circuit generates all four minterms from two inputs, and
implements the 2-4 decoder.
Larger decoders can be implemented in the same way. Here is a 3-8
Traffic Lights with a
Using a 2-4 decoder, the circuit which generates traffic light
combinations is as follows.
We no longer have to think about the problem of invalid inputs being
presented to the circuit.
To complete the traffic light controller, we just need to make the
inputs i0 and i1 cycle through the binary representations of
the numbers 0¼3. We will see how to do this later in the
Decoders of various sizes are available as standard components.
Exercise The smallest possible decoder is a 1-2. How is this
Exercise How many components (inverters and 2-input AND
gates) are needed to build an n-2n decoder?
Decoders with Enable
A standard decoder typically has an additional input called Enable.
Output is only generated when the Enable input has value 1; otherwise,
all outputs are 0. Only a small change in the implementation is
required: the Enable input is fed into the AND gates which produce
Many components have an Enable input which works in this
way. Sometimes the Enable input is ``active high'', sometimes ``active
Exercise How must the circuit be modified to make the Enable
input active low?
A multiplexer is a device which allows one of a number of inputs to be
routed to a single output. Here is a 4-1 multiplexer.
The control inputs c0 and c1 represent a 2-bit binary
number, which determines which of the inputs i0¼i3 is
connected to the output d.
If c1c0 represents the number n in binary, then the value of
the output d is the value of input in.
Multiplexers are useful in many situations. For example, in a CPU,
data being written to memory might come from one of a number of
sources - from a register, from the result of a calculation, etc -
so a multiplexer would be used to select data from the appropriate source.
Another application is where we want to be able to choose one of
several operations to carry out on some data - all the operations
can be calculated, and a multiplexer can be used to select the desired
result (more on this later).
The implementation of a multiplexer is straightforward, and
uses a decoder. Here is a 4-1 multiplexer.
All the outputs of the decoder are 0, apart from one. The inputs
c1c0 determine which of the outputs is non-zero.
All but one of the AND gates have 0 on one input and therefore
output 0. The remaining AND gate has 1 on one input and in
(where n is represented in binary by c1c0) on the other
input. The output of this AND gate is the value of in.
The OR gate has 0 on all of its inputs apart from one, and has the
value of in on the remaining input. The output of the OR gate
is therefore the value of in.
Larger multiplexers can be implemented in the same way.
A Multiplexer Application
A multiplexer and a decoder can be used together to allow sharing of a
data transmission line by a number of signals. In the following
diagram, the Control input consists of n wires, and there are 2n
data inputs and outputs. The Control input determines which of the
data inputs is connected to the transmission line.
Exercise What does a 2-1 multiplexer do? How is it
Exercise How many control inputs does a 16-1 multiplexer
The basic 2n-1 multiplexer is a switch, allowing one of 2n
inputs to be connected to the output. Each input consists of a single
It is often necessary to consider a group of wires as a single
signal. For example, in a 32-bit microprocessor, all data is handled
in blocks of 32 bits, which means that 32 wires are needed to carry a
value from one part of the circuit to another.
A collection of wires which form a single signal is called a
bus. In circuit diagrams, a bus is represented by a single line
with a short diagonal line across it, labelled to indicate the
width (number of wires) of the bus.
It is often necessary to use multiplexers to switch whole buses. In
diagrams, we simply draw a multiplexer as usual, with buses of
specified width as inputs and output.
This example shows a 4-1 multiplexer on a 32 bit bus. Note that the
control inputs are still individual wires.
A 32 bit multiplexer can be implemented with 32 basic multiplexers,
all sharing the same control inputs.
Any logic function of n inputs can be implemented with a 2n-1
multiplexer. For example, for a 2 input logic function, call the
inputs x and y and the result r, and let the truth table be:
Logic Functions (1)
where a, b, c and d are each either 0 or 1. The following
circuit implements this logic function:
because x and y, when connected to the control inputs, select the
correct row of the truth table.
Exercise How many logic gates are used by this implementation
(if the multiplexer is fully expanded into gates)? How does this
compare with the number of gates required to implement a logic
Exercise Show how each of the functions AND , OR and
NOT can be implemented with a 2-1 multiplexer.
Any logic function of 3 inputs can be implemented with a 4-1
multiplexer and an inverter, as follows.
Logic Functions (2)
Let the inputs be x, y and z. Connect x and y to the control
inputs of the multiplexer. For each combination of values of x and
y, one of the following must apply.
For each combination of values of x and y, the multiplexer input
which is selected by that combination is connected to either 0, 1,
z or [`z], depending on which of the above cases applies.
- The output is 0, regardless of the value of z.
- The output is 1, regardless of the value of z.
- The output is equal to z.
- The output is equal to [`z].
Example: Majority voting
Using a multiplexer we can build a circuit which allows one of a
number of operations to be chosen, and applied to the inputs. For
example, here is a circuit which gives a choice between AND and
If c = 1 then d = x AND y. If c = 0 then d = x OR y.
For a choice between more operations, a larger multiplexer can be
The same idea can be used for operations on multibit words. For
example, using 8 bit words, we just replace every wire (except the c
wire) by an 8 bit bus:
In this circuit, the AND operation is extended to 8 bit words by
operating on each bit position independently (and similarly OR ):
for example 11010010 AND 01110110 = 01010010.
A similar example, which is relevant to the exercises in Lab Session
3, is calculating either x AND y or x AND (NOT y), where again
x and y are multibit values.
These examples show how the ALU (arithmetic and logic unit) of a
microprocessor can be implemented. The ALU is a component which can
carry out a range of calculations on its inputs, including various
standard arithmetic and logic operations. The choice of operation is
made by setting control inputs appropriately; ultimately, the control
inputs are determined by the instruction which the CPU is executing at
any given time. We'll go further into this idea later in the course.
A demultiplexer is the opposite of a multiplexer. There is one data
input, whose value appears on one of the data outputs, depending on
the value of the control inputs. Here is a 1-4 demultiplexer.
If the control inputs c1c0 represent the number n in binary,
then the value of i is copied to output dn. Depending on the
details of the electronic implementation, the other outputs might be
0, or might be in a disconnected state.
It is straightforward to implement a demultiplexer; the circuit uses a
decoder in a similar way to the implementation of a multiplexer.
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On 27 Jul 2001, 10:19.