Page 108 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 6: Implementation of Combinational Logic Circuit

Notes
Questions:
1. Explain the XOR and XNOR gates.

2. Explain the operations in Boolean logic gate

Self Assessment
Multiple choice questions:
5. How many outputs are on a BCD decoder?

(a) 4 (b) 16
(c) 8 (d) 10
6. Use the weighting factors to convert the following BCD numbers to binary.

0101 0011 0010 0110 1000
(a) 01010011 001001101000 (b) 11010100 100001100000
(c) 110101 100001100 (d) 101011 001100001
7. What control signals may be necessary to operate a 1-line-to-16 line decoder?
(a) flasher circuit control signal (b) a LOW on all gate enable inputs
(c) input from a hexadecimal counter (d) a HIGH on all gate enable circuits

6.5 Summary

• Combinational circuit consists of input variables, logic gates and output variables. The
logic gates accept signals from the inputs and generate the outputs which are different logic
function combinations of the inputs.
• The logic gates accept signals from the inputs and generate the outputs which are different
logic function combinations of the inputs.
• Multiplexers can also be used in a more general way to synthesize logic functions.
• In the demultiplexer the address inputs determine which data output is going to have the
same value as the data input, the other data outputs will have the value 0.
• The demultiplexer is the inverse of the multiplexer, in that it takes a single data input and
n address inputs.

6.6 Keywords

Combinational logic: It is a type of digital logic which is implemented by Boolean circuits, where
the output is a pure function of the present input only.
Demultiplexer: This is the inverse of the multiplexer, in that it takes a single data input and n
address inputs. It has 2 outputs:
n
Multiplexer: This circuit has a number of data inputs, one or more select inputs, and one output.
It passes the signal value on one of the data inputs to the output.

Shannon’s expansion: Shannon’s expansion or the Shannon decomposition is a method by which
a Boolean function can be represented by the sum of two sub-functions of the original.

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