1. 5.5 Encoders
A encoder is a multiple-input, multiple-output logic circuit that converts coded inputs into coded outputs, where the input and output codes are different. The output code generally has fewer bits than the input code, and there is one-to-one mapping from input code words into output code words.
input
code word
output
enable
inputs
map
Encoder
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2. 5.5 Encoders
Binary encoder
The most common decoder circuit is an 2n-to-n encoder or binary encoder. Such a encoder has an 2n-bit binary input code and n-bit output binary code.
Binary encoder
I Y0
I1 Y1
I :
: :
: Yn-1
I2n-1
Write the truth table for 8-to-3 binary encoder and the output logic function expressions.
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3. 5.5 Encoders
Simulation
Suppose the inputs I2 and I4 are both 1, what is the output ?
If multiple requests can be made simultaneously, the encoder gives undesirable results.
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4. 5.5 Encoders Priority Encoders The 74x148 Priority Encoders
The solution is to assign priority to the input lines, so that when multiple inputs are asserted, the encoding device produces the number of the highest-priority input. Such a device is called a priority encoder.
The 74x148 Priority Encoders EI
I A2
I A1
I A0 I4
I GS
I EO I1 I0
74X148 6 7 9 1 14 15 5 4 3 2 13 12 11 10
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5. 5.5 Encoders Input data I0-I7 Enable input EI
Output GS is asserted when the device is enabled and one or more of the request are asserted.
Output EO is asserted if EI is assert but no request input is asserted, it designed to be connected to the EI input of another ‘148 that handles lower-priority requests.
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6. 5.5 Encoders Truth table for a 74x148 8-input priority encoder Inputs
Outputs
EI I0 I1 I2 I3 I4 I5 I6 I7
A2 A1 A0 GS EO
1 x x x x x x x x
0 x x x x x x x 0
0 x x x x x x
0 x x x x x
0 x x x x
0 x x x
0 x x
0 x
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7. 5.5 Encoders Logic diagram for the 74x148 priority encode
(p379 Figure 5-50)
Cascading Binary Encoders (p380 Figure 5-51)
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