AR# 10631: 3.5 FPGA Express - FPGA Express is not implementing the SRL16 properly.
AR# 10631
3.5 FPGA Express - FPGA Express is not implementing the SRL16 properly.
Description
Keywords: SRL, 16, SRL16, shift, 3.5
Urgency: Hot
General Description: Express 3.5 (build 3.5.0.5825) will implement shift registers incorrectly in Virtex/-E/-II and Spartan-II devices. Regardless of the length of the shift register you have coded, it utilizes the full 16-clock-cycle delays of the inferred SRL16 component, even if your code has been written to infer a smaller shift register.
The file is called FPGAExp35.exe and the "Posted" date is 1/5/2001.
Please be advised that this update fixes this problem when synthesizing the design through the FPGA Express, Foundation or ISE GUI. However, the problem still exists when FPGA Express is run through the FE_Shell scripting tool. The scripting problem will be corrected in a future build of FPGA Express.
To determine if you have the updated version or not, open FPGA Express by selecting:
Start -> Programs -> Xilinx Foundation Series (ISE) -> Accessories -> FPGA Express. Select Help -> About FPGA Express.
The build number for the corrected version of software is 3.5.0.6013.
To install the updated version, follow the installation instructions provided in the Readme file on the Service Pack web page mentioned above. You should reinstall the entire Service Pack as detailed in the Readme, not just the FPGA Express portion.
If you need to use fe_shell for scripting, then you will need to instantiate the SRL length that you want; to get a fixed-length shift register, drive the A3 through A0 inputs with static values. The length of the shift register can vary from 1 bit to 16 bits, as determined through use of the following formula:
Length = (8*A3) +(4*A2) + (2*A1) + A0 +1
If A3, A2, A1, and A0 are all zeros (0000), the shift register is one bit long. If they are all ones (1111), it is 16 bits long.
VHDL Example:
library ieee; use ieee.std_logic_1164.all;
entity srl_instantiation is port (D : in STD_LOGIC; CLK : in STD_LOGIC; Q : out STD_LOGIC); end entity;
architecture srl_arch of srl_instantiation is
component SRL16 port (D : in std_logic; CLK : in std_logic; A0 : in std_logic; A1 : in std_logic; A2 : in std_logic; A3 : in std_logic; Q : out std_logic); end component;