描述

How do I create 4X clocks using Virtex-E CLKDLLs?

解决方案

Virtex-E has more CLKDLLs than Virtex has. There is a total of 8; 4 primary CLKDLLs, and 4 secondary CLKDLLs. The secondary CLKDLL has a dedicated feedback loop from the 2X output to the FB pin. Therefore, generating a 4X clock is required to use only 1 BUFG instead of using 2, as in creating 4X clocks in Virtex devices.  

For more information on CLKDLL, please see (Xilinx XAPP132): "Using the Virtex Delay-Locked Loop." 

The following is an example VHDL code of how to create a 4X clock in Virtex-E: 

library ieee; 

use ieee.std_logic_1164.all; 

entity useclk is 

port ( clock, reset, din : in std_logic; 

locked, dout: out std_logic); 

end useclk; 

architecture useclk_arch of useclk is 

component BUFG port (I: in std_logic; O: out std_logic); 

end component; 

component IBUFG port (I: in std_logic; O: out std_logic); 

end component; 

component SRL16 port (A0, A1, A2, A3, CLK, D : in std_logic; Q : out std_logic); 

end component; 

component CLKDLL  

port ( 

CLKIN, CLKFB, RST : in std_logic; 

CLK0, CLK90, CLK180, CLK270, CLK2X, CLKDV, LOCKED : out std_logic); 

end component; 

signal clk_int, clk, clk_2x, clk_2x_bufg, clk_4x, clk_4x_bufg : std_logic; 

signal lock1st, lock1st_out, b, c, d, e, f, g, h, i, gd: std_logic; 

begin 

gd<='0'; 

J1 : IBUFG port map (I=>clock, O=>clk); 

U1 : CLKDLL  

port map( 

CLKIN=>clk,  

RST=>gd, 

CLKFB=>clk_2x, 

CLK0=>clk_int, 

CLK90=>b, 

CLK180=>c, 

CLK270=>d, 

CLKDV=>e, 

CLK2X=>clk_2x, 

LOCKED=>lock1st); 

U2 : SRL16 port map( 

A0=>gd, 

A1=>gd, 

A2=>gd, 

A3=>gd, 

CLK=>clk_2x, 

D=>lock1st, 

Q=>lock1st_out); 

U3 : CLKDLL port map( 

CLKIN=>clk_2x,  

RST=>not lock1st_out, 

CLKFB=>clk_4x_bufg, 

CLK0=>clk_int, 

CLK90=>f, 

CLK180=>g, 

CLK270=>h, 

CLKDV=>i, 

CLK2X=>clk_4x, 

LOCKED=>locked); 

U0 : BUFG port map (I=>clk_4x, O=>clk_4x_bufg); 

process (clk_4x_bufg, reset) 

begin 

if (reset='1') then 

dout<='0'; 

elsif (clk_4x_bufg'event and clk_4x_bufg='1') then 

dout<=din; 

end if; 

end process; 

end useclk_arch;