Hello,
I am trying to figure out how to select a w-bit(w is some parameter, shorter than the bus) part of a bus based on an incoming variable x with the smallest resource footprint possible, in one clock tick. I have tried two methods so far.
The first uses a shifter:
...
parameter bw=96
parameter w=8
...
input wire [$clog2(bw):0] x,
input wire [bw-1:0] inputWire,
output reg [w-1:0] out
...
always@(posedge clk) out=inputWire>>x;
At the second I use a for loop and multiplexers:
integer i;
always@(posedge clk)begin
for (i =0; i<bw-w;i=i+1) begin:myforloop
if(x==i) out=inputWire[i+:w];
end
end
I was quite surprised that the second method used much less resources(about half) than the first. Is there any reason for this? The only one I can come up with is that the second one covers a smaller range, but it still does not explain why the difference is this large. Also a single instance of the shifter used around 110 ALMs which is much more than I anticipated. Simpler adders take up like 4; even if the input is large like 96 it shouldn't take up so much space, should it?
Is there any better/more compact method to dynamically select part of a bus?
