在制作数字时钟之前，先要熟悉 用vhdl实现74LS161十进制计数器http://www.51hei.com/mcu/2233.html
然后在编写时钟描述语言
Library ieee;
Use ieee.std_logic_1164.all;
Use ieee.std_logic_unsigned.all;
Entity timer24 is
port(CP,CR:IN STD_LOGIC;
AS0,BS0,CS0,DS0,ES0,FS0,GS0,AS1,BS1,CS1,DS1,ES1,FS1,GS1:OUT STD_LOGIC;
AM0,BM0,CM0,DM0,EM0,FM0,GM0,AM1,BM1,CM1,DM1,EM1,FM1,GM1:OUT STD_LOGIC;
AH0,BH0,CH0,DH0,EH0,FH0,GH0,AH1,BH1,CH1,DH1,EH1,FH1,GH1:OUT STD_LOGIC);
END TIMER24;
architecture TIMER of TIMER24 is
signal H1,H0,M1,M0,S1,S0:std_logic_vector(3 downto 0);
SIGNAL C0,C1,C2,C3,C4,C5: std_logic ;
COMPONENT court161 IS
port( clk,CTT,CTP,LD,CR:in std_logic;
D3,D2,D1,D0: in std_logic;
Q:out std_logic_VECTOR(3 DOWNTO 0);
Co: out std_logic);
end COMPONENT;
COMPONENT QDLED7 IS
PORT(DATA:IN STD_LOGIC_VECTOR(3 DOWNTO 0);
a,b,c,d,e,f,g:out std_logic);
end COMPONENT;
begin
U0:court161 PORT MAP(CP,'1','1','1',CR,'0','0','0','0',S0,C0);
U1:court161 PORT MAP(C0,'1','1',(NOT(S1(2)AND S1(0))),CR,'0','0','0','0',S1,C1);
U2:court161 PORT MAP((NOT(S1(2)AND S1(0))),'1','1','1',CR,'0','0','0','0',M0,C2);
U3:court161 PORT MAP(C2,'1','1',(NOT(M1(2)AND M1(0))),CR,'0','0','0','0',M1,C3);
U4:court161 PORT MAP(C3,'1','1','1',(CR AND(NOT(H1(1) AND H0(2)))),'0','0','0','0',H0,C4);
U5:court161 PORT MAP(C4,'1','1',(NOT H1(1)),(CR AND(NOT(H1(1) AND H0(2)))),'0','0','0','0',H1,C5);
U6:QDLED7 PORT MAP(S0,AS0,BS0,CS0,DS0,ES0,FS0,GS0);
U7:QDLED7 PORT MAP(S1,AS1,BS1,CS1,DS1,ES1,FS1,GS1);
U8:QDLED7 PORT MAP(M0,AM0,BM0,CM0,DM0,EM0,FM0,GM0);
U9:QDLED7 PORT MAP(M1,AM1,BM1,CM1,DM1,EM1,FM1,GM1);
U10:QDLED7 PORT MAP(H0,AH0,BH0,CH0,DH0,EH0,FH0,GH0);
U11:QDLED7 PORT MAP(H1,AH1,BH1,CH1,DH1,EH1,FH1,GH1);
END;
接上数码管，和脉冲信号发生器，必要时要分频，分成脉冲周期为1秒的脉冲周期方可。数码管的驱动程序: http://www.51hei.com/mcu/2234.html
一般电路里的晶振频率都很高，在设计秒表或钟表时，需要将频率很高的脉冲变成频率为1Hz的脉冲，就可作为计时的标准单位，这个过程叫分频，下面是将6MHz的频率分频为周期是1秒的分频电路程序，即用2个计数200和一个计数150的三个计数器串联而成，这样200*200*150=6000000，即三个合起来计数6000000次刚好为1秒：
Library ieee;
Use ieee.std_logic_1164.all;
Use ieee.std_logic_unsigned.all;
entity fenpin6M is
port(cp:in std_logic;
clk:out std_logic);
end fenpin12M;
Architecture fen of fenpin12M is
signal Q0,Q1,Q2:std_logic_vector(7 downto 0);
signal c0,c1,c2:std_logic;
begin
process(cp,c0,c1)
begin
if cp'event and cp='1' then
if Q0<199 then Q0<=Q0+1;
else Q0<="00000000";
end if;
if Q0=199 then c0<='1';
else c0<='0';
end if;
end if;
if c0'event and c0='1' then
if Q1<149 then Q1<=Q1+1;
else Q1<="00000000";
end if;
if Q1=149 then c1<='1';
else c1<='0';
end if;
end if;
if c1'event and c1='1' then
if Q2<199 then Q2<=Q2+1;
else Q2<="00000000";
end if;
if Q2=199 then c2<='1';
else c2<='0';
end if;
end if;
end process;
clk<=c2;
end;
24小时数字时钟的VHDL程序
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