AR# 8144

Description

General Description:

CLKDLL is a clock delay-locked loop used to minimize clock skew. The simplest configuration of clockdll (BUFGDLL) can be inferred with a Xilinx-specific attribute in Synplify (Xilinx Solution 688).

Otherwise, CLKDLL needs to be instantiated as described below. For further information on CLKDLL usages, please reference (Xilinx XAPP132).

For an example on how to pass attribute to CLKDLL, please see (Xilinx Solution 11095). This solution illustrates DCM instantiation, but the attribute passing method is the same.

NOTE: Tested in Synplify 5.2.2a.

The example uses the default I/O standard (LVTTL). If using other I/O standards, instantiate the appropriate IBUFG_selectIO.

Solution

Verilog example

In this example, the ACLK's frequency is doubled and used inside and outside the chip. BCLK and OUTBCLK are connected in the board outside the chip.

`include "<path_to>/unisim.v"

module clock_test(ACLK, DIN, QOUT, BCLK, OUTBCLK, BCLK_LOCK, RESET);

input ACLK, BCLK;

input RESET;

input [1:0] DIN;

output [1:0] QOUT;

output OUTBCLK, BCLK_LOCK;

reg [1:0] QOUT;

IBUFG CLK_ibufg_A

(.I (ACLK),

.O(ACLK_ibufg)

);

BUFG ACLK_bufg

(.I (ACLK_2x),

.O (ACLK_2x_design)

);

IBUFG CLK_ibufg_B

(.I (BCLK), // connected to OUTBCLK outside

.O(BCLK_ibufg)

);

CLKDLL ACLK_dll_2x // 2x clock

(.CLKIN(ACLK_ibufg),

.CLKFB(ACLK_2x_design),

.RST(1'b0),

.CLK2X(ACLK_2x),

.CLK0(),

.CLK90(),

.CLK180(),

.CLK270(),

.CLKDV(),

.LOCKED(ACLK_lock)

);

CLKDLL BCLK_dll_OUT // off-chip synchronization

(.CLKIN(ACLK_ibufg),

.CLKFB(BCLK_ibufg), // BCLK and OUTBCLK is connected outside the chip.

.RST(1'b0),

.CLK2X(OUTBCLK), //connected to BCLK outside

.CLK0(),

.CLK90(),

.CLK180(),

.CLK270(),

.CLKDV(),

.LOCKED(BCLK_LOCK)

);

always @(posedge ACLK_2x_design or posedge RESET)

begin

if (RESET)

QOUT[1:0] <= 2'b00;

else if (ACLK_lock)

QOUT[1:0] <= DIN[1:0];

end

endmodule

VHDL example

library IEEE;

use IEEE.std_logic_1164.all;

library unisim;

use unisim.vcomponents.all;

entity CLOCK_TEST is

port(

ACLK : in std_logic;

-- off chip feedback, connected to OUTBCLK on the board.

BCLK : in std_logic;

--OUT CLOCK

OUTBCLK : out std_logic;

DIN : in std_logic_vector(1 downto 0);

RESET : in std_logic;

QOUT : out std_logic_vector (1 downto 0);

-- CLKDLL lock signal

BCLK_LOCK : out std_logic

);

end CLOCK_TEST;

architecture RTL of CLOCK_TEST is

component IBUFG

port (

I : in std_logic;

O : out std_logic);

end component;

component BUFG

port (

I : in std_logic;

O : out std_logic);

end component;

component CLKDLL

port (

CLKIN : in std_logic;

CLKFB : in std_logic;

RST : in std_logic;

CLK0 : out std_logic;

CLK90 : out std_logic;

CLK180 : out std_logic;

CLK270 : out std_logic;

CLKDV : out std_logic;

CLK2X : out std_logic;

LOCKED : out std_logic);

end component;

-- Glock signals

signal ACLK_ibufg : std_logic;

signal BCLK_ibufg : std_logic;

signal ACLK_2x : std_logic;

signal ACLK_2x_design : std_logic;

signal ACLK_lock : std_logic;

begin

ACLK_ibufg : IBUFG

port map (

I => ACLK,

O => ACLK_ibufg

);

BCLK_ibufg : IBUFG

port map (

I => BCLK,

O => BCLK_ibufg

);

ACLK_bufg : BUFG

port map (

I => ACLK_2x,

O => ACLK_2x_design

);

ACLK_dll : CLKDLL

port map (

CLKIN => ACLK_ibufg,

CLKFB => ACLK_2x_design,

RST => '0',

CLK2X => ACLK_2x,

CLK0 => OPEN,

CLK90 => OPEN,

CLK180 => OPEN,

CLK270 => OPEN,

CLKDV => OPEN,

LOCKED => ACLK_lock

);

BCLK_dll_out : CLKDLL

port map (

CLKIN => ACLK_ibufg,

CLKFB => BCLK_ibufg,

RST => '0',

CLK2X => OUTBCLK,

CLK0 => OPEN,

CLK90 => OPEN,

CLK180 => OPEN,

CLK270 => OPEN,

CLKDV => OPEN,

LOCKED => BCLK_lock

);

process (ACLK_2x_design, RESET)

begin

if RESET = '1' then

QOUT <= "00";

elsif ACLK_2x_design'event and ACLK_2x_design = '1' then

if ACLK_lock = '1' then

QOUT <= DIN;

end if;

end if;

end process;

END RTL;