FPGA学习心得——矩阵键盘

1、行列式键盘概述

为了减少键盘与单片机接口时所占用I/O口线的数目，在键数较多时，通常都将键盘排列成行列矩阵式，行列式键盘又叫矩阵式键盘。用带有I/O口的线组成行列结构，按键设置在行列的交点上。例如用2*2的行列结构可以构成4个键的键盘，4*4的行列结构可以构成有16个键的键盘。这样，当按键数量平方增长时，I/O口线只是线性增长，这样就可以节省I/O口线。

2、行列式键盘原理

教研室已有薄膜矩阵键盘，其实物图如图所示。

其电路原理图如下图所示。

按键设置在行列线交叉点，行、列线分别连接到按键开关的两端。列线通过上拉电阻接+5V的电压，即列线的输出被钳位到高电平状态。行线与按键的一个引脚相连，列线与按键的另一个引脚相连。判断键盘中有无按键按下是通过行线送入扫描信号，然后从列线读取状态得到的。其方法是依次给行线送低电平，检查列线的输入。如果列线信号全为高电平，则代表低电平信号所在的行中无按键按下；如果列线有输入为低电平，则低电平信号所在的行和出现低电平的列的交点处有按键按下。

设行扫描信号为keydrv3~keydrv0，列线按键输入信号keyin3~keyin0与按键位置的关系如下表所示。

由行列式键盘的原理可以知道，要正确地完成按键输入工作必须有按键扫描电路产生keydrv3~keydrv0信号。同时还必须有按键译码电路从keydrv3~keydrv0信号和keyin3~keyin0信号中译码出按键的键值。此外，一般还需要一个按键发生标志信号用于和其他模块接口，通知其它模块键盘上有按键动作发生，并可以从键盘模块中读取按键键值。由于各个模块需要的时钟频率是不一样的，因此时钟产生模块就是用于产生各个模块需要的时钟信号。因此得到键盘接口电路的结构如图2所示。

图2 键盘接口电路结构图

行列式键盘电路的FPGA实现主要解决三个问题，一是如何检测是否有按键按下并防止采集到干扰信号；二是在按键闭合时如何防止抖动；三是如何判断为哪一个按键位动作，并对其进行译码。因此，为了解决这些问题，程序中使用不同的进程分别实现键盘扫描信号的产生、键盘去抖以及键盘的译码。

3、源程序

------------------------------------------------------------------------------------ Company: -- Engineer: -- -- Create Date:    08:46:57 07/31/2012 -- Design Name: -- Module Name:    MatrixKeyboard - Behavioral -- Project Name: -- Target Devices: -- Tool versions: -- Description: ---- Dependencies: ---- Revision: -- Revision 0.01 - File Created-- Additional Comments: ------------------------------------------------------------------------------------library IEEE;use IEEE.STD_LOGIC_1164.ALL;use IEEE.STD_LOGIC_ARITH.ALL;use IEEE.STD_LOGIC_UNSIGNED.ALL;---- Uncomment the following library declaration if instantiating---- any Xilinx primitives in this code.--library UNISIM;--use UNISIM.VComponents.all;entity MatrixKeyboard is    Port ( Clk : in  STD_LOGIC;  Reset: inSTD_LOGIC;           KeyIn : in  STD_LOGIC_VECTOR (3 downto 0);  KeyScan : out   STD_LOGIC_VECTOR (3 downto 0);           LED : out  STD_LOGIC_VECTOR (3 downto 0)  );end MatrixKeyboard;architecture Behavioral of MatrixKeyboard isSignal Clk_scan:STD_LOGIC:='0';Signal Clk_5ms:STD_LOGIC:='0';Signal Clk_2ms:STD_LOGIC:='0';Signal Key_Scan:  STD_LOGIC_VECTOR(3 downto 0);Signal Key_Decode: STD_LOGIC_VECTOR(7 downto 0);Type State_Key is(st_key1,st_key2,st_key3,st_key4);Signal Current_Key : State_Key := st_key1;Type State_Scan is(st_scan1,st_scan2,st_scan3,st_scan4);Signal Current_Scan : State_Scan := st_scan1;beginProc_Clk_5ms : process(Clk)variable cnt_clk : integer range 0 to 250000 := 0;beginif(rising_edge(Clk)) thenif(cnt_clk < 125000) thencnt_clk := cnt_clk + 1;Clk_scan<= '0';elsif(cnt_clk < 249999) thencnt_clk := cnt_clk + 1;Clk_scan<= '1';elsecnt_clk := 0;end if;Clk_5ms<= Clk_scan;end if;end process Proc_Clk_5ms;Proc_Clk_2ms : process(Clk)variable cnt_clk : integer range 0 to 100000 := 0;beginif(rising_edge(Clk)) thenif(cnt_clk < 50000) thencnt_clk := cnt_clk + 1;Clk_2ms<= '0';elsif(cnt_clk < 99999) thencnt_clk := cnt_clk + 1;Clk_2ms<= '1';elsecnt_clk := 0;end if;end if;end process Proc_Clk_2ms;Proc_Scan:process(Clk_5ms)beginif(rising_edge(Clk_5ms)) thencase Current_Scan iswhen st_scan1 =>Key_Scan <= "1110";Current_Scan <= st_scan2;when st_scan2 =>Key_Scan <= "1101";Current_Scan <= st_scan3;when st_scan3 =>Key_Scan <= "1011";Current_Scan <= st_scan4;when st_scan4 =>Key_Scan <= "0111";Current_Scan <= st_scan1;end case;end if;end process Proc_Scan;KeyScan <= Key_Scan;Key_Decode <= Key_Scan & Keyin;Proc_Keyboard:process(Clk_2ms,Reset)variable cnt_btn : integer range 0 to 50000 := 0;beginif(Reset = '1') thenLED<=x"1";Current_Key<= st_key1;elsif(falling_edge(Clk_2ms)) thencase Current_Key iswhen st_key1 =>--Check whether any keys are pressedif((Keyin and "1111") = "1111") then Current_Key <= st_key1;elseCurrent_Key <= st_key2;end if;when st_key2 =>--keys debouncingif((Keyin and "1111") = "1111") then Current_Key <= st_key1;elsecase Key_Decode iswhen "11101110" => LED <= "0001";when "11101101" => LED <= "0010";when "11101011" => LED <= "0011";when "11100111" => LED <= "1010";when "11011110" => LED <= "0100";when "11011101" => LED <= "0101";when "11011011" => LED <= "0110";when "11010111" => LED <= "1011";when "10111110" => LED <= "0111";when "10111101" => LED <= "1000";when "10111011" => LED <= "1001";when "10110111" => LED <= "1100";when "01111110" => LED <= "1110";when "01111101" => LED <= "0000";when "01111011" => LED <= "1111";when "01110111" => LED <= "1101";                                     when others     => null;     end case;end if;Current_Key <= st_key3;when st_key3 =>--Check whether the pressed keys are releasedif((Keyin and "1111") /= "1111") then Current_Key <= st_key3;elseCurrent_Key <= st_key4;end if;when st_key4 =>--keys debouncingif((Keyin and "1111") /= "1111") then Current_Key <= st_key3;elseLED<=x"0";Current_Key <= st_key1;end if;end case;end if;end process Proc_Keyboard;end Behavioral;