4-to-2 Priority Encoder

Information

A 4-to-2 Priority Encoder is a digital circuit that encodes the highest-order active input (logic ‘1’) among four inputs into a 2-bit binary output. If multiple inputs are active, the highest-priority input determines the output.

Priority order: In3 > In2 > In1 > In0

Truth Table

In3	In2	In1	In0	Out1	Out0	Valid
0	0	0	0	0	0	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

Ports

• In0–In3: Input lines (active high)

• Out0, Out1: 2-bit encoded binary output

• Valid: Output signal indicating if any input is active (optional)

Model

The Encoder4to2 model implements a 4-input to 2-output priority encoder.

Attributes:

• In0–In3 (dsignal): Four digital input lines

• Out0, Out1 (dsignal): Two binary output lines

• Valid (dsignal): Indicates if an input is active (1), else 0

Methods:

digital(): Outputs the binary representation of the highest active input

from pyams.lib import dsignal, model

class Encoder4to2(model):
    """ 4-to-2 Priority Encoder """
    def __init__(self, In0, In1, In2, In3, Out0, Out1, Valid):
        self.In0 = dsignal(direction='in', port=In0)
        self.In1 = dsignal(direction='in', port=In1)
        self.In2 = dsignal(direction='in', port=In2)
        self.In3 = dsignal(direction='in', port=In3)

        self.Out0 = dsignal(direction='out', port=Out0)
        self.Out1 = dsignal(direction='out', port=Out1)
        self.Valid = dsignal(direction='out', port=Valid)

    def digital(self):
        """ Perform priority encoding """
        i0, i1, i2, i3 = self.In0, self.In1, self.In2, self.In3
        self.Valid += i0 | i1 | i2 | i3
        self.Out1 += i2 | i3
        self.Out0 += i1 | i3

Command syntax

The syntax for defining a 4-to-2 priority encoder in a PyAMS simulation:

# Import the model
from pyams.models import Encoder4to2

# EN: instance name
# In0–In3: inputs; Out0–Out1: encoded outputs; Valid: activity flag
EN = Encoder4to2(In0, In1, In2, In3, Out0, Out1, Valid)