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Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 3
Outline
• Decoder • 1 of 2 and 1 of 4 line decoder• 1 of 8 • 4 of 16 line decoder• Function specific decoders
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 4
Decoder• A decoder is a circuit that selects the
binary information from a set of inputs• A decoder selects a unique
combination of inputs according to the information and generate output at a unique output at a line (or lines)
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 5
Decoder• Assume that we have 2n circuits to
implement different functions. One is for addition; other is for subtraction; other for incrementing; and so on. We have to select only one by giving appropriate input.
• A 1 of 2n decoder will let us select only one using n-bit input. For example, we want to decode and get a selected output when the input is 0001 (like an instruction) and activate the addition circuit, one of 16 circuits
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 6
Decoder• Assume that we have 10 digits, which
corresponds to 7 segments of LEDs in an LED display unit. We have to select only one unique set of outputs by giving a unique set of input bits.
• A 7-segment decoder will let us select only one. For example, we want to decode and get a selected output when 4-bit input is 1000 and decoder activates all seven segments of the LEDs to display digit 8.
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 7
1 to n Decoder
• A circuit, which takes the n-bit input, activates only one of the output out of 2n outputs and remaining remains inactive.
• Inactive output can be taken as tristate or 1 or 0
• Active output be taken as 1 or 0
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 8
Outline
• Decoder • 1 of 2 and 1 of 4 line decoders• 1 of 8 line decoder• 4 of 16 line decoder• Function specific decoders
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 9
1 of 2 Decoder with active 0 output
0 0 *
Inputs Outputs A Y0 Y1
1 * 0
* means tristate or 1
Y0Y1
A
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 10
1 of 2 Decoder with active 1 output
0 1 *
Inputs Outputs A Y0 Y1
1 * 1
* means tristate or 0
Y0Y1
A
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 11
1 of 4 Decoder with active 0 output
0 0 0 * * *
Inputs Outputs A1 A0 Y0 Y1 Y2 Y3
0 1 * 0 * *1 0 * * 0 *1 1 * * * 0
* means tristate or 1
Y1Y0
Y2Y3
A0A1
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 12
Outline
• Decoder • 1 of 2 and 1 of 4 line decoders• 1 of 8 line decoder• 4 of 16 line decoder• Function specific decoders
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 13
1 of 8 Decoder with active 0 output
0 0 0 0 *** *
Inputs Outputs A2 A1 A0 Y0 ..... Y7
.. .. .. .. ..
1 1 1 * *** 0
* means tristate or 1
...Y0
Y7
A0A1A2
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 14
1 of 8 Decoder with three control (enabling/disabling) pins
X X X
≠≠≠≠100 ***** *
Inputs Outputs
A2 A1 A0 Y0 ...Y70 0 0 100 0 *** * 1 1 1 100 *** 0
When G2 = 1, the G1 and G0 both 0s, then only one of the Y output activates. X means 1 or 0.
ControlInputs
G2G1G0
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 15
MSI 74138 —1 of 8 Decoder with three control (enabling/disabling) pins
Y0
Y7
G2G1G0
Y1Y2
Y3Y4
Y5Y6
A0A1A2
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 16
Outline
• Decoder • 1 of 2 and 1 of 4 line decoders• 1 of 8 line decoder• 4 line of 16 line decoder • Function specific decoders
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 17
MSI 74154 —1 of 16 (4 line to 16 line) Decoder with three control (enabling/disabling) pins
Y0
Y15
A3
G1G0
Y1Y2
Y14
A0A1A2 .
.
.
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 18
Outline
• Decoder • 1 of 2 and 1 of 4 line decoders• 1 of 8 line decoder• 4 of 16 line decoder• Function specific decoders
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 19
BCD to Decimal line decoder
MSIIC 7442
A4A3A2A1A0
Y0
Y9
Y1Y2
Y0
.
Y10
Active means 1Inactive means 0
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 20
Multi-line decoder BCD to 7-segment LED display
MSIIC 7447
A3A2A1A0
Y0
e
bc
a
g
Active means 0Inactive means 1
d
f
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 22
Decoder
• Decoder active one output pin among 2n output pins and has n inputs to enable circuit to select corresponding output.
• Specific function decoder gives outputs on set of pins as per the given set of inputs
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 23
Decoder
• A decoder can have inactive outputs in tristate and active output as 0 or 1
• A decoder can have control gate pins
Ch10L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 24
End of Lesson 2 on
Decoder