Priority Encoder
A priority Encoder encodes one of the many inputs but only the key having the highest priority is encoded first. If highest priority key is not pressed then the next lower priority key is encoded and so on.
The functional table is given below, In the table Y0 and Y1 are the encoded output, output V is an indicator of a valid key press and is high when one of the valid key is pressed:
| K0 | K1 | K2 | K3 | Y0 | Y1 | V |
| 0 | 0 | 0 | 0 | x | x | 0 |
| 1 | 0 | 0 | 0 | 0 | 0 | 1 |
| x | 1 | 0 | 0 | 0 | 1 | 1 |
| x | x | 1 | 0 | 1 | 0 | 1 |
| x | x | x | 1 | 1 | 1 | 1 |
The first row in table shows none of the key pressed , so the output (Y0, Y1) are don’t care, and V output is zero indicating none of the valid key pressed.
The other rows shows only one input is active and the least numbered keys are don’t cared, and a valid key press entry V is raised and corresponding binary output obtained at Y1, Y1.
The Boolean equations
Figure: SOP Simplification of Y0 and Y1
The equation for V is:
V = K0 + K1 + K2 + K3
The logic diagram for the 4 to 2 priority encoder is shown in figure below:
Figure: Priority Encoder Circuit
VHDL Code for 4:2 priority encoder
[tabby title=”Dataflow Modeling Priority Encoder”]
VHDL Code for 4:2 priority encoder
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
entity PRIORITY_ENCODER is
Port (
K : in STD_LOGIC_VECTOR (3 downto 0);
Y : out STD_LOGIC_VECTOR (1 downto 0);
V : out STD_LOGIC
);
end PRIORITY_ENCODER;
architecture Behavioral of PRIORITY_ENCODER is begin
Y(0) <= K(2) OR K(3);
Y(1) <= (K(3) OR (K(1) AND (not K(2))));
V <= K(0) OR K(1) OR K(2) OR K(3);
end Behavioral;
ISIM Simulation
Figure: Priority Encoder Simulation
[tabby title=”Structure Modeling Priority Encoder”]
Structural Modelling of 4:2 Priority Encoder
entity or_4ip is
Port (
i : in STD_LOGIC_VECTOR (3 downto 0);
o : out STD_LOGIC
);
end or_4ip;
architecture Behavioral of or_4ip is
begin
o <= i(0) or i(1) or i(2) or i(3);
end Behavioral;
entity OR_GATE is
Port (
i1 : in STD_LOGIC;
i2 : in STD_LOGIC;
o : out STD_LOGIC
);
end OR_GATE;
architecture Behavioral of OR_GATE is
begin
o <= i1 or i2;
end Behavioral;
entity NO_GATE is
Port (
i : in STD_LOGIC;
o : out STD_LOGIC
);
end NO_GATE;
architecture Behavioral of NO_GATE is
begin
o <= not i;
end Behavioral;
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
library my_logics_lib;
use my_logics_lib.all;
entity encoder_structure is
Port (
k : in STD_LOGIC_VECTOR (3 downto 0);
o : out STD_LOGIC_VECTOR (1 downto 0);
v : out STD_LOGIC
);
end encoder_structure;
architecture Behavioral of encoder_structure is
signal w1, w2: std_logic;
begin
x1: entity work.or_gate port map (
i1=>k(3), i2 => w1, o => o(1)
);
x2: entity work.NO_gate port map (
i=> k(2), o => w2
);
x3: entity work.and_gate port map (
i1=>k(1), i2 => w2, o => w1
);
x4: entity work.or_gate port map (
i1=>k(2), i2 => k(3), o => o(0)
) ;
x5: entity work.or_4ip port map (
i(0)=> k(0), i(1)=> k(1), i(2) => k(2), i(3) => k(3), o => v
);
end Behavioral;
RTL Synthesis
Figure: Encoder RTL
[tabbyending]
