Congestion hotspots often come from many-to-one muxes, massive decode trees, and centralized control. Distribute decisions where possible.
Localize muxing and decode close to data origin.
Evaluate pipelining against diagnosed timing paths and the architectural latency budget.
Prefer structured hierarchy over huge monolithic logic clouds.
An 8:1 mux can pull eight buses toward one spot. Factoring into two 4:1 muxes plus a 2:1 suggests local source grouping, but synthesis can flatten or merge the structure. Placement regions and supported preservation controls may be needed; inspect mapped drivers, routing congestion and timing under identical flow settings before claiming a benefit.
Level | Selected by | Chooses |
|---|---|---|
First, near each source group |
| One of four words per group |
Second, at the meeting point |
| Between the two group winners |
Read it top-down. A flat 8:1 mux pulls all eight buses to one spot; this split sends only two buses to the meeting point, if the tool keeps the structure and places each 4:1 near its group.
No. They suggest local grouping, but mapped topology and placement decide whether routes stay local. Compare routed congestion and timing, including any new final-stage bottleneck.
Every bus must route to the one place where the mux sits, so wires crowd around it and lengthen. Select within each nearby group first and mux the few group winners, then check the routed result.
The two-level mux checked against a flat reference with data rotations and independent walking-one/walking-zero patterns on every source. Each selected data bit must follow the selected source, including the low nibble.
`timescale 1ns/1ps
module mux2level (
input logic [7:0] src0,
input logic [7:0] src1,
input logic [7:0] src2,
input logic [7:0] src3,
input logic [7:0] src4,
input logic [7:0] src5,
input logic [7:0] src6,
input logic [7:0] src7,
input logic [2:0] sel,
output logic [7:0] y
);
logic [7:0] grp_lo, grp_hi;
always_comb begin
case (sel[1:0])
2'd0: grp_lo = src0;
2'd1: grp_lo = src1;
2'd2: grp_lo = src2;
default: grp_lo = src3;
endcase
case (sel[1:0])
2'd0: grp_hi = src4;
2'd1: grp_hi = src5;
2'd2: grp_hi = src6;
default: grp_hi = src7;
endcase
case (sel[2])
1'b0: y = grp_lo;
default: y = grp_hi;
endcase
end
endmodule