Avoiding latches

How to avoid numorous latches?

Suppose you're building a circuit to process scancodes from a PS/2 keyboard for a game. Given the last two bytes of scancodes received, you need to indicate whether one of the arrow keys on the keyboard have been pressed. This involves a fairly simple mapping, which can be implemented as a case statement (or if-elseif) with four cases.

Scancode [15:0]	Arrow key
16'he06b	left arrow
16'he072	down arrow
16'he074	right arrow
16'he075	up arrow
Anything else	none

Your circuit has one 16-bit input, and four outputs. Build this circuit that recognizes these four scancodes and asserts the correct output.

To avoid creating latches, all outputs must be assigned a value in all possible conditions (See also always_if2). Simply having a default case is not enough. You must assign a value to all four outputs in all four cases and the default case. This can involve a lot of unnecessary typing. One easy way around this is to assign a "default value" to the outputs before the case statement:

always @(*) begin
    up = 1'b0; down = 1'b0; left = 1'b0; right = 1'b0;
    case (scancode)
        ... // Set to 1 as necessary.
    endcase
end

This style of code ensures the outputs are assigned a value (of 0) in all possible cases unless the case statement overrides the assignment. This also means that a default: case item becomes unnecessary.

Reminder: The logic synthesizer generates a combinational circuit that behaves equivalently to what the code describes. Hardware does not "execute" the lines of code in sequence.

Practice

Problem statement

// synthesis verilog_input_version verilog_2001
module top_module (
    input [15:0] scancode,
    output reg left,
    output reg down,
    output reg right,
    output reg up  ); 

endmodule

Solution

// synthesis verilog_input_version verilog_2001
module top_module (
    input [15:0] scancode,
    output reg left,
    output reg down,
    output reg right,
    output reg up  ); 
    always @(*) begin
        up = 1'b0; down = 1'b0; left = 1'b0; right = 1'b0;
        case (scancode)
            16'he06b: left=1;
            16'he072: down=1;
            16'he074: right=1;
            16'he075: up=1;
        endcase
    end
endmodule