How can i make my verilog shifter more general? - verilog

Question

Here i have a shifter but as of rite now it only works for up to 3 bits. I've been looking and i can't find out how to make it work for up to 8 bits.
module shifter(a,b,out);
input [7:0] a, b;
output [7:0] out;
wire [7:0] out1, out2, out3;
mux_8b_2to1 first(a[7:0], {a[3:0],a[7:4]}, b[2], out1);
mux_8b_2to1 second(out1[7:0], {out1[5:0],out1[7:6]}, b[1], out2);
mux_8b_2to1 third(out2[7:0], {out2[6:0],out2[7]}, b[0], out);
endmodule

Answer 1

What you have is a Barrel Shifter. Two ways to make it more generic are make it a functional model (still synthesis-able) or structural model with a generate block. Both approaches follow IEEE Std 1364-2001 (aka Verilog-2001).
The functional generic approach for a barrel shifter only needs a down-shifter. The general function is out = {in,in} >> (WIDTH-shift) where leftover bits can be ignored. To protect for double-roll (i.e. shift > WIDTH ), use the mod operator on the shift (WIDTH-(shift%WIDTH)).
module barrel_shifter_functional #( parameter CTRL=3, parameter WIDTH=CTRL**2 )
( input wire [WIDTH-1:0] in,
input wire [ CTRL-1:0] shift,
output wire [WIDTH-1:0] out );
assign out = {2{in}} >> (WIDTH-(shift%WIDTH));
endmodule
The structural generic approach for a barrel shifter needs a generate block. The for loop in the generate block will unravel at compile time, not run time like a for loop like in an always block. To keep it generic also have have the 2-to-1 mux have a parametrized width. FYI, you can use the generate block with functional code too, for example comment out the mux_2to1 instantiation and uncomment the assign statement below it. Learn more about the generate block by reading IEEE Std 1800-2012 § 27. Generate constructs.
module barrel_shifter_structeral #( parameter CTRL=3, parameter WIDTH=CTRL**2 )
( input wire [WIDTH-1:0] in,
input wire [ CTRL-1:0] shift,
output wire [WIDTH-1:0] out );
wire [WIDTH-1:0] tmp [CTRL:0];
assign tmp[CTRL] = in;
assign out = tmp[0];
genvar i;
generate
for (i = 0; i < CTRL; i = i + 1) begin : mux
mux_2to1 #(.WIDTH(WIDTH)) g(
.in0(tmp[i+1]),
.in1({tmp[i+1][WIDTH-(2**i)-1:0],tmp[i+1][WIDTH-1:WIDTH-(2**i)]}),
.sel(shift[i]),
.out(tmp[i]) );
// assign tmp[i] = shift[i] ? {tmp[i+1][WIDTH-(2**i)-1:0],tmp[i+1][WIDTH-1:WIDTH-(2**i)]} : tmp[i+1];
end : mux
endgenerate
endmodule
module mux_2to1 #( parameter WIDTH=8 )
( input wire [WIDTH-1:0] in0, in1,
input wire sel,
output wire [WIDTH-1:0] out );
assign out = sel ? in1 : in0;
endmodule
Both examples are functionally equivalent and synthesize provided CTRL is less than or equal to the ceiling of log2(WIDTH). Synthesis will likely give different results. The generate method will exclusively use 2-to-1 muxes while the pure functional method will depend on the quality of the optimizer.
Working example # http://www.edaplayground.com/s/6/500

Answer 2

I've used the >> and << operators to generate a synthetizable design using ISEWebPack, as this:
module shifter(
input wire [7:0] a,
input wire [7:0] b,
input wire leftright, // 0=shift right, 1=shift left
output reg [7:0] out
);
always #* begin
if (leftright==0)
out = a>>b;
else
out = a<<b;
end
endmodule
This way, the symthesis tool will know that you want to implement a shifter and can use its own macros to best synthetize it:
Synthesizing Unit <shifter>.
Related source file is "shifter.v".
Found 8-bit shifter logical right for signal <out$shift0002> created at line 30.
Found 8-bit shifter logical left for signal <out$shift0003> created at line 32.