Key Pad

Theory

A matrix keypad is a set of buttons arranged in rows and columns (e.g., 4x3 or 4x4). Instead of having a separate wire for each button, the matrix design reduces the number of I/O pins required. 

diagram

 verilog code

module keypad_scanner (

    input clk,

    input reset,

    input [3:0] col,    // Columns from keypad

    output reg [3:0] row,   // Rows to keypad

    output reg [3:0] key,   // 4-bit key code

    output reg key_valid    // High when a key is detected

);

    reg [1:0] scan_row;

    reg [19:0] debounce;

    always @(posedge clk or posedge reset) begin

        if (reset) begin

            scan_row <= 0;

            row <= 4'b1110;

            debounce <= 0;

            key_valid <= 0;

        end else begin

            debounce <= debounce + 1;

            if (debounce == 0) begin

                scan_row <= scan_row + 1;

                row <= ~(1 << scan_row);  // Activate one row at a time

            end

            case (row)

                4'b1110: if (col != 4'b1111) decode_key(0, col);

                4'b1101: if (col != 4'b1111) decode_key(1, col);

                4'b1011: if (col != 4'b1111) decode_key(2, col);

                4'b0111: if (col != 4'b1111) decode_key(3, col);

            endcase

        end

    end

    task decode_key;

        input [1:0] row_num;

        input [3:0] col_val;

        begin

            key_valid = 1;

            case (col_val)

                4'b1110: key = row_num * 4 + 0;

                4'b1101: key = row_num * 4 + 1;

                4'b1011: key = row_num * 4 + 2;

                4'b0111: key = row_num * 4 + 3;

                default: begin key = 4'b0000; key_valid = 0; end

            endcase

        end

    endtask

endmodule

pin assignment:

Pin clock--P56

Pin reset--P57

Key  0--P58

Key  1--P59

Key  2--P60

Key  3--P61

row<0>-P62

row<1>-P63

row<2>-P64

row<3>-P65

Device utilization:

Time analysis: