rotation - Verilog Barrel Shifter

Question

I want to create a 64-bit barrel shifter in verilog (rotate right for now). I want to know if there is a way to do it without writing a 65 part case statement? Is there a way to write some simple code such as:

    Y = {S[i - 1:0], S[63:i]};

I tried the code above in Xilinx and get an error: i is not a constant.

Main Question: Is there a way to do this without a huge case statment?

Answer 1

I've simplified some of the rules for clarity, but here are the details.

In the statement

Y = {S[i - 1:0], S[63:i]};

you have a concatenation of two signals, each with a constant part select. A constant part select is of the form

identifier [ constant_expression : constant_expression ]

but your code uses a variable for the first expression. As you saw this isn't allowed, but you are correct in that there are ways to avoid typing a large case statement. What you can use instead is an indexed part select. These are of the form

identifier [ expression +: constant_expression ]

identifier [ expression -: constant_expression ]

These constructs enforce that the width of the resulting signal is constant, regardless of the variable on the left side.

wire [HIGH_BIT:LOW_BIT] signalAdd,signaSub;
signalAdd[some_expression +: some_range];
signalSub[some_expression -: some_range];
//Resolves to
signalAdd[some_expression + (some_range - 1) : some_expression];
signalSub[some_expression                    : some_expression - (some_range - 1)];

//The location of the high value depends on how the signal was declared:
wire [15: 0] a_vect;
wire [0 :15] b_vect;
a_vect[0 +: 8] // a_vect[7 : 0]
b_vect[0 +: 8] // b_vect[0 : 7]

Rather than trying to build one signal out of two part selects, you can simply extend the input signal to 128 bits, and use a variable part select from that.

wire [63:0] data_in,data_out;
wire [127:0] data_in_double;
wire [5:0] select;

//Concatenate the input signal
assign data_in_double = {data_in,data_in};

//The same as signal[select + 63 : select]
assign data_out = data_in_double[select+63-:64];

Another approach you could use is generate loops. This is a more general approach to replicating code based on a variable. It is much less efficient since it creates 4096 signals.

wire [63:0] data_in,data_out;
wire [127:0] data_in_double;
wire [5:0] select;
wire [63:0] array [0:63];
genver i;

//Concatenate the input signal
assign data_in_double = {data_in,data_in};
for(i=0;i<64;i=i+1)
  begin : generate_loop
  //Allowed since i is constant when the loop is unrolled
  assign array[i] = data_in_double[63+i:i];
  /*
  Unrolls to 
  assign array[0] = data_in_double[63:0];
  assign array[1] = data_in_double[64:1];
  assign array[2] = data_in_double[65:2];
  ...
  assign array[63] = data_in_double[127:64];
  */
  end

//Select the shifted value
assign data_out = array[select];