What does a priority encoder do when several inputs are active?

Outputs the code of the lowest input

What does the V output indicate?

At least one input is active, V = D3+D2+D1+D0

Which input has highest priority

2080 Baisakh · 2+4 marks

Priority encoder; 4-to-2 priority encoder logic

Short answer

A priority encoder outputs the code of the highest-priority active input with a valid bit V. For 4-to-2 (D3 highest): Y1=D3+D2, Y0=D3+D2' D1, V=D3+D2+D1+D0. Circuit uses OR, AND, NOT gates.

What is a priority encoder? Find out logic expressions and draw the logic circuit of 4 to 2 priority encoder.

A priority encoder is an encoder that gives the binary code of the highest-priority active input when more than one input is active at the same time. It also has a valid output $V$ that shows whether any input is active (an ordinary encoder gives a wrong code when two inputs are high).

4-to-2 priority encoder

Inputs $D_0$ – $D_3$ , with $D_3$ the highest priority. Outputs $Y_1Y_0$ and $V$ (X = don't care).

$D_3$	$D_2$	$D_1$	$D_0$	$Y_1$	$Y_0$	$V$
0	0	0	0	X	X	0
0	0	0	1	0	0	1
0	0	1	X	0	1	1
0	1	X	X	1	0	1
1	X	X	X	1	1	1

Expressions (from K-maps of the expanded table):

$Y_1 = 1$ when $D_3 = 1$ or $D_2 = 1$ :

Y_1 = D_3 + D_2

$Y_0 = 1$ when $D_3 = 1$ , or when $D_1 = 1$ with $D_2 = 0$ (and $D_3=0$ ):

Y_0 = D_3 + D_3'D_2'D_1 = D_3 + D_2'D_1

$V$ is 1 if any input is 1:

V = D_3 + D_2 + D_1 + D_0

Logic circuit

D3 -----------+---------> [OR] --> Y1
D2 ---+-------|---------> [  ]
      |       |
      +-[NOT]-|-+
              | [AND]-+
D1 -----------|-[   ] +-> [OR] --> Y0
              +---------> [  ]

D0,D1,D2,D3 ----------> [4-in OR] --> V

So the circuit needs one NOT, one 2-input AND, two 2-input OR gates and one 4-input OR gate. Example: $D_3D_2D_1D_0 = 0110$ gives $Y_1Y_0 = 10$ because $D_2$ has higher priority than $D_1$ .

✓ Answer checked by Claude Opus 5.5 (high effort)

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