问一个关于dpll的问题
在其他论坛上下了一个有问题的dpll程序，由于以前没有设计过pll，所以现在连port的意义及用处都不明白，所以想请各位介绍一下，谢谢。程序如下：
Library ieee;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_arith.all;
use IEEE.std_logic_unsigned.all;
entity dpll is
port(
datain: instd_logic; -- raw data input
clock: instd_logic; -- 64*bit clock
rx_clock: out std_logic; -- recovered rx clock
dataout:out std_logic; -- received data output
clrdcd: instd_logic; -- clear dcd when 8 ones are detected in hdlcdec
dcd: out std_logic); -- data carrier detect output
end dpll;
architecture behaviour of dpll is
signal counter: std_logic_vector(4 downto 0); -- counter 0..31
signal dcd_cntr: std_logic_vector(5 downto 0);-- counter 0..127
signal edge: std_logic; -- edge detector output: data decision changed
signal dly_data: std_logic; -- delayed data for edge detector
signal ql: std_logic; -- late clock
signal qe: std_logic; -- early clock
signal enable: std_logic; -- gets toggled every clock or when clock has to be adjusted
signal increment: std_logic;
signal decrement: std_logic;
signal clear_dcd: std_logic;
signal reset_dcd: std_logic;
begin
--
-- recovered rx clock for followning stages
--
rx_clock <= counter(4);
process(clock,clrdcd,reset_dcd)
begin
if (clock'event and clock='1' ) then
clear_dcd <= reset_dcd or clrdcd;
end if;
end process;
--
-- clock in new data
--
process(clock,datain)
begin
if (clock'event and clock='1' ) then
dataout <= datain;
end if;
end process;
--
-- rx clock counter
--
process(clock,enable,clrdcd)
begin
--if ( clrdcd='1') then -- reset for illegal data at hdlcdec
-- counter <= (others => '0');
if (clock'event and clock='1' ) then
if (enable='1') then
counter <= counter+1; -- increase counter
else
counter <= counter;
end if;
end if;
end process;
--
-- set early and late clocks
--
process(counter)
begin
if (counter="10000" or counter ="01111" ) then
ql <= '0';
qe <= '0';
elsif (counter(4)='1') then-- late clock when counter > 32
ql <= '1';
qe <= '0';
else
ql <= '0'; -- early clock when counter < 31
qe <= '1';
end if;
end process;
--
-- adjst rx clock
-- this is done by changing the enable signal for the rx clock counter
-- enable gets toggled every clock event. It gets set additionally when
-- the clock counter should be incremented and cleared when clock should
-- be decremented.
--
process(clock,enable,clrdcd)
begin
--if ( clrdcd='1') then
-- enable<='0';
-- increment <='0';
-- decrement <='0';
if (clock'event and clock='1') then
if (qe='1' and edge='1') then
increment <='1'; -- increment clock when edge detect
end if; -- during early clock
if (ql='1' and edge='1') then
decrement <='1'; -- decrement clock when edge detect
end if; -- during late clock
if (enable ='1') then
if (increment ='1') then
increment <='0'; -- clear after one increment step
enable <='1';
else
enable <='0';
end if;
else
if (decrement ='1') then
decrement<='0'; -- clear after one decrement step
enable <= '0';
else
enable <='1';
end if;
end if;
end if;
end process;
--
-- dcd detection
--
process(clock,edge,counter,clear_dcd)
begin
if ( clear_dcd='1') then
dcd_cntr<= (others => '0');
dcd <= '0';
reset_dcd <='0';
elsif (counter(4)'event and counter(4)='0') then
if ( edge='0' ) then -- sample at rising edge, if no data change increase counter
if (dcd_cntr = 63) then
dcd <= '1'; -- assert dcd if dcd counter is at max
dcd_cntr <= dcd_cntr;
else
dcd <= '0';
dcd_cntr <= dcd_cntr+1;
end if;
else
reset_dcd <='1';
end if;
end if;
end process;
--
-- edge detector, input data has changed
--
process(clock,datain)
begin
if (clock'event and clock='1') then
edge <= dly_data xor datain;
dly_data <= datain;
end if;
end process;
end behaviour;
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