74ls00设计计时器

根据提供的引用内容，我们可以了解到74LS192和74LS48D是设计计时器电路的重要元件。下面是一个基于74LS192和74LS48D设计的计时器电路的示例：

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity timer is
    Port ( clk : in  STD_LOGIC;
           reset : in  STD_LOGIC;
           start : in  STD_LOGIC;
           stop : in  STD_LOGIC;
           out1 : out  STD_LOGIC_VECTOR (3 downto 0);           out2 : out  STD_LOGIC_VECTOR (3 downto 0));
end timer;

architecture Behavioral of timer is

signal count : std_logic_vector(3 downto 0) := "0000";
signal temp : std_logic_vector(3 downto 0) := "0000";
signal flag : std_logic := '0';

begin

process(clk, reset)
begin
    if reset = '1' then
        count <= "0000";
        temp <= "0000";
        flag <= '0';
    elsif rising_edge(clk) then
        if start = '1' then
            flag <= '1';
        end if;
        if flag = '1' then
            temp <= temp + 1;
            if temp = "1010" then
                temp <= "0000";
                count <= count + 1;
            end if;
        end if;
        if stop = '1' then
            flag <= '0';
        end if;
    end if;
end process;

out1 <= count;
out2 <= temp;

end Behavioral;

该计时器电路包含两个输出，out1和out2，分别表示计时器的分钟和秒钟。计时器的开始和停止由start和stop信号控制，计时器的复位由reset信号控制。计时器的时钟信号由clk提供。