Mux & demux

Multiplexers, demultiplexers, and select-path coding styles.

A multiplexer selects one of several data inputs. A demultiplexer steers one input onto one of several outputs (often with one-hot enables).

Mux styles in RTL

Ternary (small):

assign y = sel ? a : b;

case (readable, scales):

always_comb begin
  unique case (sel)
    2'b00: y = d0;
    2'b01: y = d1;
    2'b10: y = d2;
    default: y = d3;
  endcase
end

Priority mux (if/else chain): earlier branches win. Use when selects are not mutually exclusive; otherwise prefer unique case / one-hot.

One-hot vs binary select

Select	Pros	Cons
Binary	Compact control	Decode cost; illegal codes need `default`
One-hot	Fast mux in some libraries; clear enables	Wider select bus; must stay one-hot

If software/firmware can send illegal sel, define a safe default (hold previous registered value, or a known idle).

Demux pattern

always_comb begin
  y0 = '0; y1 = '0; y2 = '0; y3 = '0;
  unique case (sel)
    2'd0: y0 = din;
    2'd1: y1 = din;
    2'd2: y2 = din;
    default: y3 = din;
  endcase
end

Clearing all outputs first avoids latches when only one lane is driven.

Synthesis notes

Nested ternaries become mux trees — fine, but watch timing on wide / deep selects.
Don’t mix blocking and non-blocking in the same combo process.
For registered outputs, mux in combo then flop: always_ff @(posedge clk) q <= y_next;.