Page 91 - Handout Digital Electronics

LECTURE 11 COMBINATIONAL LOGIC CIRCUIT BUILDING BLOCKS
11.1 Decoders
A decoder is a combinational logic circuit with several output lines one of which is active at any one
time. The active output line is dependent on the pattern of the input lines. In general, a decoder has no
n
input lines and 2 output lines. Below is a logic circuit of a 2 to 4 decoder.

Figure 23: 2 to 4 Decoder logic circuit

The Boolean expressions for the outputs F0 – F3 are as follows:

Fo = enable.AB

F1 = enable.AB
F 2 = enable.AB
F 3 = enable.AB

The truth table for the above decoder can be produced by tracing the input signals from input to output
noting the respective values of each output line or can be produced by substituting the binary values of
each input combination in the above Boolean expressions.

enable = 1
A B F0 F1 F2 F3
0 0 1 0 0 0
0 1 0 1 0 0
1 0 0 0 1 0
1 1 0 0 0 1

Decoders have many uses in digital computers. They can be used to control other circuits. For example,
if no changes are needed in other circuits, the enable line can simply be set to zero (0). The other
example where decoders are used is in memory address decoding. For example, to construct a 1 KB
memory using four (4) 256 x 8 RAM chips, there is need for a single unified address space which can be
broken down as follows:

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