基于PIC32的PS/2键盘通讯协议(MCU to keyboard)

                         MCU到Keyboard的通讯协议

 

注:MCU到Keyboard的通讯主要用在控制键盘上的LED灯(NumLock,CapsLock和ScrollLock)

 

1.首先介绍下PS/2接口和发送的帧位:

   如图1和图2所示,PS/2的接口定义如下:

     

                         图1.PS/2接口定义

 

 

               

                          图2.PS/2接口母座定义

 

   图3为PS/2串行通讯协议中每帧11位数据的定义,如下图所示:

   

                      图3.PS/2每帧11位的串行协议

 

2.PS/2协议的详细通讯过程

    PS/2设备总是产生时钟信号.如果主机要发送数据,它必须首先把时钟和数据线设置为"请求发送"状态.可以通过下拉时钟线至少100微秒来抑制通讯,通过下拉数据线来应用"请求发送",然后释放时钟.设备应该在不超过10毫秒的间隔内就要检查这个状态.当设备检测到这个状态,它将开始产生时钟信号,并且时钟脉冲标记下输入八个数据位和一个停止位.主机仅当时钟线为低的时候改变数据线,而数据在时钟脉冲的上升沿被锁存.这在发生在设备到主机的通讯的过程中正好相反.在停止位发送后不释放数据线,设备将继续产生时钟脉冲直到数据线被释放(然后设备将产生一个错误).

    要使得这个过程易于理解,主机必须按下面的步骤发送数据到PS/2设备:

1.把时钟线拉低至少100微秒.

2.把数据线拉低.

3.释放数据线.

4.等待设备把时钟线拉低.

5.设置/复位数据线发送第一个数据位.

6.等待设备把时钟线拉高.

7.等待设备把时钟线拉低.

8.重复5~7步骤,发送剩下的7个数据位和校验位.

9.释放数据线.

10.等待设备把数据线拉低.

11.等待设备把时钟线拉低.

12.等待设备释放数据线和时钟线.

 

                                图4.主机到设备的详细通讯过程

 

 

3.如下是Debug版的Host to Device的PS/2通讯代码:

    其中有很多处return,主要是用于观测返回值,看看通讯出错的地方.另外TIMER_10ms[4]是一个定时器,防止程序死机用的.如果调试成功也可以把这些都去掉.

#define  PS2KB_SendByte_Success    0
BYTE _PS2KB_SendByte(BYTE iData)
{
     BYTE i;
     BYTE temp;
     BYTE tempC;
     BOOL flag_check = 1;

       temp = iData;
       for(i = 0; i < 8; i++)
     {
            tempC = temp & 0x01;
            if(tempC == 0x01)
                flag_check = !flag_check;
            temp = temp >> 1;
     }
       PD_PS2_CLK_TRIS = 0;

       PD_PS2_CLK_LAT = 0;

       DELAY_Us(100); //Bring the Clock line low for at least 100 microseconds
       PD_PS2_DATA_TRIS = 0;

       PD_PS2_DATA_LAT = 0;//Bring the Data line low

       DELAY_Us(20);
       PD_PS2_CLK_TRIS = 1; //Release the Clock line.


       temp = iData;
       for(i = 0; i < 8; i++)
     {
            tempC = temp & 0x01;

            while( TIMER_10ms[4]&& PD_PS2_CLK_IO ); //Wait for the device to bring the Clock line low
            if( ! TIMER_10ms[4] ) return (i+i+1);

            if(tempC == 0x01) //Set/reset the Data line to send the data bit
                PD_PS2_DATA_LAT = 1;
            else
                PD_PS2_DATA_LAT = 0;
            temp = temp >> 1;
            while( TIMER_10ms[4]&& !PD_PS2_CLK_IO ); //Wait for the device to bring Clock high.
            if( ! TIMER_10ms[4] ) return (i+i+2);


     }
       while( TIMER_10ms[4]&& PD_PS2_CLK_IO ); //Wait for the device to bring the Clock line low
       if( ! TIMER_10ms[4] ) return 17;

       if(flag_check == 0x01) //Set/reset the Data line to send the parity bit
             PD_PS2_DATA_LAT = 1;
     else
            PD_PS2_DATA_LAT = 0;

       while( TIMER_10ms[4]&& !PD_PS2_CLK_IO ); //Wait for the device to bring the Clock line high.
       if( ! TIMER_10ms[4] ) return 18;

       while( TIMER_10ms[4]&& PD_PS2_CLK_IO ); //Wait for the device to bring the Clock line low
       if( ! TIMER_10ms[4] ) return 19;

       PD_PS2_DATA_TRIS = 1; //Release the Data line

       while( TIMER_10ms[4]&& PD_PS2_DATA_IO ); //Wait for the device to bring Data low
       if( ! TIMER_10ms[4] ) return 20;

       while( TIMER_10ms[4]&& PD_PS2_CLK_IO ); //Wait for the device to bring Clock low
       if( ! TIMER_10ms[4] ) return 21;


       while( TIMER_10ms[4]&& (!PD_PS2_DATA_IO || !PD_PS2_CLK_IO) ); //Wait for the device to release Data and Clock
       if( ! TIMER_10ms[4] ) return 22;


       // the following is to receive the ack data

       while( TIMER_10ms[4]&& PD_PS2_CLK_IO ); //Wait for the device to bring the Clock line low
       if( ! TIMER_10ms[4] ) return (23);

       if(PD_PS2_DATA_IO) return (24);//read the Data line to check the start bit

       while( TIMER_10ms[4]&& !PD_PS2_CLK_IO ); //Wait for the device to bring Clock high.
       if( ! TIMER_10ms[4] ) return (25);

       flag_check = 1;

       for(i = 0; i < 8; i++)
     {

            while( TIMER_10ms[4]&& PD_PS2_CLK_IO ); //Wait for the device to bring the Clock line low
            if( ! TIMER_10ms[4] ) return (i+i+26);

            if(PD_PS2_DATA_IO) flag_check = !flag_check;

            temp |= ((((BYTE)PD_PS2_DATA_IO) & 0x01) << i);

            while( TIMER_10ms[4]&& !PD_PS2_CLK_IO ); //Wait for the device to bring Clock high.
            if( ! TIMER_10ms[4] ) return (i+i+27);

     }

       if( temp != PS2_KEYCMD_ACK )
       {
             return 42;
       }

       while( TIMER_10ms[4]&& PD_PS2_CLK_IO ); //Wait for the device to bring the Clock line low
       if( ! TIMER_10ms[4] ) return 43;

       if( flag_check) return 44; //read the Data line to check the parity bit

       while( TIMER_10ms[4]&& !PD_PS2_CLK_IO ); //Wait for the device to bring Clock high.
       if( ! TIMER_10ms[4] ) return 45;


       while( TIMER_10ms[4]&& PD_PS2_CLK_IO ); //Wait for the device to bring the Clock line low
       if( ! TIMER_10ms[4] ) return 46;

       if(!PD_PS2_DATA_IO) return 47; //read the Data line to check the stop bit

       while( TIMER_10ms[4]&& !PD_PS2_CLK_IO ); //Wait for the device to bring Clock high.
       if( ! TIMER_10ms[4] ) return 48;
       return PS2KB_SendByte_Success;
}

 

 

                          键盘LED灯无法点亮问题

   

    尽管有了上面的程序但是我之前也未能实现键盘上LED灯的点亮(有些键盘可以,但有些键盘不行),这个问题困扰了我很久,偶然的机会配到一个资深的工程师帮我解决了这个问题,问题如下:

     图1为DR100上PS/2接口部分的电路.因为DR100用的是TTL的电平,所以DR100管脚输出的高电平为3.3V.即PS/2 DATA端和PS/2 CLK端的电平都为3.3V.而PS/2 VCC(即C点电位)为5V.通过计算可得A点与B点的电位约为3.41V.但是由于小键盘采用的是CMOS电平,而CMOS的高电平是定义为大于3.5V的电平,所以无法驱动小键盘.

  

             图5.PS/2接口电路

待我把R216和R219的电阻短路后后便可以点亮LED灯了.