Description

General Description: 

A dynamically variable shift register LUT (SRL) is an SRL that can vary the shift amount based on the address inputs of the SRL. (For more information on SRLs, please refer to the Libraries Guide at http://support.xilinx.com/support/library.htm.)
 

How do I infer a dynamically variable shift register LUT for the Virtex architecture?

Solution

The following example is taken from the Synplify Pro User Guide (C:\Synplicity\synplify_70\doc), pages 4-80: 

VHDL example: 

library IEEE; 

use IEEE.std_logic_1164.all; 

entity srltest is 

port ( inData: std_logic_vector(7 downto 0); 

clk, en : in std_logic; 

outStage : in integer range 3 downto 0; 

outData: out std_logic_vector(7 downto 0)); 

end srltest; 

architecture rtl of srltest is 

type dataAryType is array(3 downto 0) of std_logic_vector(7 downto 0); 

signal regBank : dataAryType; 

begin 

outData <= regBank(outStage); 

process(clk, inData) begin 

if (clk'event and clk = '1') then 

if (en='1') then 

regBank <= (regBank(2 downto 0) & inData); 

end if; 

end if; 

end process; 

end rtl;

The following example is taken from the Synplify Pro User Guide (C:\Synplicity\synplify_70\doc), pages 4-80: 

Verilog example: 

module test_srl(clk, enable, dataIn, result, addr); 

input clk, enable; 

input [3:0] dataIn; 

input [3:0] addr; 

output [3:0] result; 

reg [3:0] regBank[15:0]; 

integer i; 

always @(posedge clk) begin 

if (enable == 1) begin 

for (i=15; i>0; i=i-1) begin 

regBank[i] <= regBank[i-1]; 

end 

regBank[0] <= dataIn; 

end 

end 

assign result = regBank[addr]; 

endmodule