interfacing the pc's keyboard.htm

使用68HC705J1A写的一个键盘的代码,包括资料说明,一份难得的资料,本人开发键盘时好不容易才找到的资料

            <TR>
              <TD vAlign=top><B>00</B></TD>
              <TD><B>Error or Buffer Overflow</B></TD></TR>
            <TR>
              <TD></TD>
              <TD></TD></TR>
            <TR>
              <TD vAlign=top>FF</TD>
              <TD>Error or Buffer 
      Overflow</TD></TR></TBODY></TABLE></CENTER></UL><BR></UL>
      <HR>
      <B><FONT size=+2>Scan Codes</FONT></B>
      <HR>
       
      <UL>
        <P>The diagram below shows the Scan Code assigned to the individual 
        keys. The Scan code is shown on the bottom of the key. E.g. The Scan 
        Code for ESC is 76. All the scan codes are shown in Hex. </P>
        <CENTER><IMG alt="Scancodes - Keyboard" 
        src="Interfacing the PC's Keyboard.files/scancode.gif"></CENTER><BR>
        <UL>
          <P>As you can see, the scan code assignments are quite random. In many 
          cases the easiest way to convert the scan code to ASCII would be to 
          use a look up table. Below is the scan codes for the extended keyboard 
          &amp; Numeric keypad. </P></UL>
        <CENTER><IMG alt="Scancodes -  Numeric Keypad" 
        src="Interfacing the PC's Keyboard.files/scancod1.gif"></CENTER><BR></UL>
      <HR>
      <B><FONT size=+2>The Keyboard's Connector</FONT></B>
      <HR>
       
      <UL>
        <P>The PC's AT Keyboard is connected to external equipment using four 
        wires. These wires are shown below for the 5 Pin DIN Male Plug &amp; 
        PS/2 Plug. </P>
        <CENTER>
        <TABLE width="70%">
          <TBODY>
          <TR>
            <TD>
              <CENTER><IMG alt="5 Pin Din Numbering (Male Plug)" 
              src="Interfacing the PC's Keyboard.files/5pindin.gif"><BR><B>5 Pin 
              DIN</B><BR></CENTER></TD>
            <TD>1. KBD Clock<BR>2. KBD Data<BR>3. N/C<BR>4. GND<BR>5. +5V 
              (VCC)<BR></TD>
            <TD>
              <CENTER><IMG alt="PS2 Numbering (Male Plug)" 
              src="Interfacing the PC's Keyboard.files/ps2.gif"><BR><B>PS/2</B><BR></CENTER></TD>
            <TD>1. KBD Clock<BR>2. GND<BR>3. KBD Data<BR>4. N/C<BR>5. +5V 
              (VCC)<BR>6. N/C <BR></TD></TR></TBODY></TABLE></CENTER>
        <P>A fifth wire can sometimes be found. This was once upon a time 
        implemented as a Keyboard Reset, but today is left disconnected on AT 
        Keyboards. Both the KBD Clock and KBD Data are Open Collector 
        bi-directional I/O Lines. If desired, the Host can talk to the keyboard 
        using these lines. </P>
        <P><I>Note: Most keyboards are specified to drain a maximum 300mA. This 
        will need to be considered when powering your devices </I></P></UL>
      <HR>
      <B><FONT size=+2>The Keyboard's Protocol</FONT></B>
      <HR>
       
      <UL><B><FONT size=+2>Keyboard to Host</FONT></B>
        <HR>
         
        <P>As mentioned before, the PC's keyboard implements a bi-directional 
        protocol. The keyboard can send data to the Host and the Host can send 
        data to the Keyboard. The Host has the ultimate priority over direction. 
        It can at anytime (although the not recommended) send a command to the 
        keyboard. </P>
        <P>The keyboard is free to send data to the host when both the KBD Data 
        and KBD Clock lines are high (Idle). The KBD Clock line can be used as a 
        Clear to Send line. If the host takes the KBD Clock line low, the 
        keyboard will buffer any data until the KBD Clock is released, ie goes 
        high. Should the Host take the KBD Data line low, then the keyboard will 
        prepare to accept a command from the host. </P>
        <P>The transmission of data in the forward direction, ie Keyboard to 
        Host is done with a frame of 11 bits. The first bit is a Start Bit 
        (Logic 0) followed by 8 data bits (LSB First), one Parity Bit (Odd 
        Parity) and a Stop Bit (Logic 1). Each bit should be read on the falling 
        edge of the clock. </P>
        <CENTER><IMG alt="Keyboard to Host Protocol Waveform" 
        src="Interfacing the PC's Keyboard.files/keyboar1.gif"></CENTER>
        <P>The above waveform represents a one byte transmission from the 
        Keyboard. The keyboard may not generally change it's data line on the 
        rising edge of the clock as shown in the diagram. The data line only has 
        to be valid on the falling edge of the clock. The Keyboard will generate 
        the clock. The frequency of the clock signal typically ranges from 20 to 
        30 Khz. The Least Significant Bit is always sent first. </P><B><FONT 
        size=+2>Host to Keyboard</FONT></B>
        <HR>
         
        <P>The Host to Keyboard Protocol is initiated by taking the KBD data 
        line low. However to prevent the keyboard from sending data at the same 
        time that you attempt to send the keyboard data, it is common to take 
        the KBD Clock line low for more than 60us. This is more than one bit 
        length. Then the KBD data line is taken low, while the KBD clock line is 
        released. </P>
        <P>The keyboard will start generating a clock signal on it's KBD clock 
        line. This process can take up to 10mS. After the first falling edge has 
        been detected, you can load the first data bit on the KBD Data line. 
        This bit will be read into the keyboard on the next falling edge, after 
        which you can place the next bit of data. This process is repeated for 
        the 8 data bits. After the data bits come an Odd Parity Bit. </P>
        <CENTER><IMG alt="Host to Keyboard - Protocol Waveform" 
        src="Interfacing the PC's Keyboard.files/keyboard.gif"></CENTER>
        <P>Once the Parity Bit has been sent and the KBD Data Line is in a idle 
        (High) state for the next clock cycle, the keyboard will acknowledge the 
        reception of the new data. The keyboard does this by taking the KBD Data 
        line low for the next clock transition. If the KBD Data line is not idle 
        after the 10th bit (Start, 8 Data bits + Parity), the keyboard will 
        continue to send a KBD Clock signal until the KBD Data line becomes 
        idle. </P></UL><A name=1></A>
      <HR>
      <B><FONT size=+2>Interfacing Example - Keyboard to ASCII 
Decoder</FONT></B>
      <HR>
       
      <UL>
        <P>Normally in this series of web pages, we connect something to the PC, 
        to demonstrate the protocols at work. However this poses a problem with 
        the keyboard. What could be possibly want to send to the computer via 
        the keyboard interface? </P>
        <P>Straight away any devious minds would be going, why not a little box, 
        which generates passwords!. It could keep sending characters to the 
        computer until it finds the right sequence. Well I'm not going to 
        encourage what could possibly be illegal practices. </P>
        <P>In fact a reasonably useful example will be given using a 68HC705J1A 
        single chip microcontroller. We will get it to read the data from the 
        keyboard, convert the scan codes into ASCII and send it out in RS-232 
        format at 9600 BPS. However we won't stop here, you will want to see the 
        bi-directional use of the KBD Clock &amp; Data lines, thus we will use 
        the keyboards status LEDS, Num Lock, Caps Lock and Scroll Lock. </P>
        <P>This can be used for quite a wide range of things. Teamed up with a 
        reasonably sized 4 line x 40 character LCD panel, you could have 
        yourself a little portable terminal. Or you could use it with a 
        microcontroller development system. The 68HC705J1A in a One Time 
        Programmable (OTP) is only a fraction of the cost of a 74C922 keyboard 
        decoder chip, which only decodes a 4 x 4 matrix keypad to binary. </P>
        <P>The keyboard doesn't need to be expensive either. Most people have 
        many old keyboards floating around the place. If it's an AT Keyboard, 
        then use it (XT keyboards will not work with this program.) If we ever 
        see the introduction of USB keyboards, then there could be many 
        redundant AT keyboards just waiting for you to hook them up. 
        </P><B><FONT size=+2>Features</FONT></B>
        <HR>
         
        <P>Before we start with the technical aspects of the project, the 
        salesman in me wants to tell you about the features packed into the 998 
        bytes of code. </P>
        <UL>
          <LI>Use of the keyboard's bi-directional protocol allowing the status 
          of the Num Lock, Caps Lock and Scroll Lock to be displayed on the 
          Keyboards LEDs. <BR><BR>
          <LI>External Reset Line activated by ALT-CTRL-DEL. If you are using it 
          with a Microcontroler development system, you can reset the MCU with 
          the keyboard. <I>I've always wanted to be able to use the three 
          fingered solute on the HC11!</I> <BR><BR>
          <LI>Scroll Lock and Num Lock toggles two Parallel Port Pins on the 
          HC705. This can be used to turn things on or off, Select Memory Pages, 
          Operating Systems etc <BR><BR>
          <LI>"ALTDEC" or what I call the Direct Decimal Enter Routine. Just 
          like using a PC, when you enter a decimal number when holding down one 
          of the ALT keys the number is sent as binary to the target system. 
          E.g. If you press and hold down ALT, then type in 255 and release ALT, 
          the value FF (Hex) will be sent to the system. <I>Note. Unlike the PC, 
          you can use both the numeric keypad or the numbers along the top of 
          the keyboard. </I><BR><BR>
          <LI>"CTRLHEX" or you guessed it, Direct Hexadecimal Enter Routine. 
          This function is not found with the PC. If you hold CTRL down, you can 
          enter a Hexadecimal number. Just the thing for Development Systems or 
          even debugging RS-232 Comms? <BR><BR>
          <LI>Output is in ASCII using a RS-232 format at 9600 BPS. If using it 
          with a development System, you can tap it in after the RS-232 Line 
          Transceivers to save you a few dollars on RS-232 Level Converters. 
          </LI></UL><BR><B><FONT size=+2>Schematic &amp; Hardware</FONT></B>
        <HR>
         
        <P>The schematic below, shows the general connections of the keyboard to 
        the HC705. </P>
        <CENTER><IMG alt="Schematic for PC Keyboard Decoder" 
        src="Interfacing the PC's Keyboard.files/keybrd05.gif"></CENTER>
        <P>The TXD pin, while it transmits in RS-232 format, is not at RS-232 
        Voltage Levels. If you want to connect it to RS-232 Devices then you 
        will need to attach a RS-232 Level Converter of some kind. If you are 
        using it with a development system, you can bypass both RS-232 Level 
        Converters and connect it directly to the RXD pin of the MCU. However 
        the keyboard can't be a direct replacement for a terminal on a 
        development system, unless you want to type in your code each time! You 
        may want to place a jumper or switch inline to switch between your 
        RS-232 Port and the Keyboard. </P>
        <P>The Keyboard requires open collector/open drain outputs. This is 
        achieved by using the Data Direction Register (DDR). A zero is written 
        to the port which is internally latched. The DDR is then used to toggle 
        the line from logic 0 to high impedance. If the port pin is an output, a 
        logic zero will be present on the pin, if the port is set to be an 
        input, the port will be high impedance which is pulled high by the 
        external resistors. </P>
        <P>The circuit is designed to run on a 4Mhz crystal (2Mhz Bus Speed). 
        The timing for the RS-232 transmission is based on the bus speed, thus 
        this crystal has to be 4 Mhz. If you are lucky enough to have a 4 Mhz E 
        Clock on your development system you can use it. </P>
        <P>The power supply can also create a slight problem. A standard 
        keyboard can drain about 300mA max, thus it would be recommended to use 
        it's own regulator rather than taking a supply from elsewhere. 
        Decoupling capacitors are not shown on the general schematic but are 
        implied for reliable operation. Consult your MC68HC705J1A Technical Data 
        Manual for more Information. </P><B><FONT size=+2>Reading Bytes from the 
        Keyboard.</FONT></B>