the ps-2 mouse-keyboard protocol.htm

PS/2接口的键盘与鼠标接口规范

  <P><IMG height=330 alt="" src="The PS-2 Mouse-Keyboard Protocol.files/ps2.jpg" 
  width=352> <BR></P></BLOCKQUOTE>
<P><BR>Note: When looking through examples on this website, you'll notice I use 
a few tricks when implementing an open-collector interface with PIC 
microcontrollers. &nbsp;I use the same pin for both input and output, and I 
enable the PIC's internal pullup resistors rather than using external resistors. 
&nbsp;A line is pulled to ground by setting the corresponding pin to output, and 
writing a "zero" to that port. &nbsp;The line is set to the "high impedance" 
state by setting the pin to input. &nbsp;Taking into account the PIC's built-in 
protection diodes and sufficient current sinking, I think this is a valid 
configuration. &nbsp;Let me know if your experiences have proved 
otherwise.<BR><BR><B>Communication: General Description</B><BR></P>
<P>The PS/2 mouse and keyboard implement a bidirectional synchronous serial 
protocol.&nbsp; The bus is "idle" when both lines are high (open-collector). 
&nbsp;This is the only state where the keyboard/mouse is allowed begin 
transmitting data. &nbsp;The host has ultimate control over the bus and may 
inhibit communication at any time by pulling the Clock line low. &nbsp;<BR></P>
<P>The device always generates the clock signal. &nbsp;If the host wants to send 
data, it must first inhibit communication from the device by pulling Clock low. 
&nbsp;The host then pulls Data low and releases Clock. &nbsp;This is the 
"Request-to-Send" state and signals the device to start generating clock 
pulses.<BR></P>
<BLOCKQUOTE>
  <P><U>Summary: Bus States</U><BR>Data = high, Clock = high: &nbsp;<I>Idle 
  state.</I><BR>Data = high, Clock = low: &nbsp;<I>Communication 
  Inhibited.</I><BR>Data = low, Clock = high: &nbsp;<I>Host 
  Request-to-Send</I></P></BLOCKQUOTE>&nbsp; All data is transmitted one byte at a 
time and each byte is sent in a frame consisting of 11-12 bits.&nbsp; These bits 
are: 
<UL>
  <LI>1 start bit.&nbsp; This is always 0. 
  <LI>8 data bits, least significant bit first. 
  <LI>1 parity bit (odd parity). 
  <LI>1 stop bit.&nbsp; This is always 1. 
  <LI>1 acknowledge bit (host-to-device communication only) </LI></UL>
<P>The parity bit is set if there is an even number of 1's in the data bits and 
reset (0) if there is an odd number of 1's in the data bits.&nbsp; The number of 
1's in the data bits plus the parity bit always add up to an odd number (odd 
parity.)&nbsp; This is used for error detection. &nbsp;The keyboard/mouse must 
check this bit and if incorrect it should respond as if it had received an 
invalid command.<BR></P>
<P>Data sent from the device to the host is read on the <I>falling </I>edge of 
the clock signal; data sent from the host to the device is read on the <I>rising 
</I>edge<I>.</I>&nbsp; The clock frequency must be in the range 10 - 16.7 kHz. 
&nbsp;This means clock must be high for 30 - 50 microseconds and low for 30 - 50 
microseconds.. &nbsp;If you're designing a keyboard, mouse, or host emulator, 
you should modify/sample the Data line in the middle of each cell. 
&nbsp;I.e.&nbsp; 15 - 25 microseconds after the appropriate clock transition. 
&nbsp;Again, the keyboard/mouse always generates the clock signal, but the host 
always has ultimate control over communication. </P>
<P></P>Timing is absolutely crucial. &nbsp;Every time quantity I give in this 
article must be followed exactly.<BR><BR><B>Communication: 
Device-to-Host</B><BR>
<P>The Data and Clock lines are both open collector. &nbsp;A resistor is 
connected between each line and +5V, so the idle state of the bus is high. When 
the keyboard or mouse wants to send information, it first checks the Clock line 
to make sure it's at a high logic level.&nbsp; If it's not, the host is 
inhibiting communication and the device must buffer any to-be-sent data until 
the host releases Clock. &nbsp;The Clock line must be continuously high for at 
least 50 microseconds before the device can begin to transmit its data.&nbsp; 
</P>
<P>As I mentioned in the previous section, the keyboard and mouse use a serial 
protocol with 11-bit frames.&nbsp; These bits are: </P>
<UL>
  <LI>1 start bit.&nbsp; This is always 0. 
  <LI>8 data bits, least significant bit first. 
  <LI>1 parity bit (odd parity). 
  <LI>1 stop bit.&nbsp; This is always 1. </LI></UL>The keyboard/mouse writes a 
bit on the Data line when Clock is high, and it is read by the host when Clock 
is low. &nbsp;Figures 2 and 3 illustrate this.<BR>
<P><FONT color=#ff0000>Figure 2:&nbsp; Device-to-host communication.&nbsp; The 
Data line changes state when Clock is high and that data is valid when Clock is 
low.</FONT> <BR></P>
<BLOCKQUOTE><IMG height=139 alt="" 
  src="The PS-2 Mouse-Keyboard Protocol.files/waveform1.jpg" width=432> 
</BLOCKQUOTE>
<P></P>
<P><FONT color=#ff0000>Figure 3:&nbsp; Scan code for the "Q" key (15h) being 
sent from a keyboard to the computer.&nbsp; Channel A is the Clock signal; 
channel B is the Data signal.</FONT> </P>
<BLOCKQUOTE><FONT color=#ffffff>---</FONT><IMG height=255 alt="" 
  src="The PS-2 Mouse-Keyboard Protocol.files/qscope.jpg" width=386> 
<BR></BLOCKQUOTE>
<P>The clock frequency is 10-16.7 kHz.&nbsp; The time from the rising edge of a 
clock pulse to a Data transition must be at least 5 microseconds.&nbsp; The time 
from a data transition to the falling edge of a clock pulse must be at least 5 
microseconds and no greater than 25 microseconds.&nbsp; <BR></P>
<P>The host may inhibit communication at any time by pulling the Clock line low 
for at least 100 microseconds. &nbsp;If a transmission is inhibited before the 
11th clock pulse, the device must abort the current transmission and prepare to 
retransmit the current "chunk" of data when host releases Clock. &nbsp;A "chunk" 
of data could be a make code, break code, device ID, mouse movement packet, etc. 
&nbsp;For example, if a keyboard is interrupted while sending the second byte of 
a two-byte break code, it will need to retransmit both bytes of that break code, 
not just the one that was interrupted.<BR></P>
<P>If the host pulls clock low before the first high-to-low clock transition, or 
after the falling edge of the last clock pulse, the keyboard/mouse does not need 
to retransmit any data. &nbsp;However, if new data is created that needs to be 
transmitted, it will have to be buffered until the host releases Clock. 
&nbsp;Keyboards have a 16-byte buffer for this purpose. &nbsp;If more than 16 
bytes worth of keystrokes occur, further keystrokes will be ignored until 
there's room in the buffer. &nbsp;Mice only store the most current movement 
packet for transmission. </P>
<P><B>Host-to-Device Communication:</B><BR></P>
<P>The packet is sent a little differently in host-to-device communication... 
</P>
<P>First of all, the PS/2 device always generates the clock signal.&nbsp; If the 
host wants to send data, it must first put the Clock and Data lines in a 
"Request-to-send" state as follows: </P>
<UL>
  <LI>Inhibit communication by pulling Clock low for at least 100 microseconds. 
  <LI>Apply "Request-to-send" by pulling Data low, then release Clock. 
</LI></UL>The device should check for this state at intervals not to exceed 10 
milliseconds.&nbsp; When the device detects this state, it will begin generating 
Clock signals and clock in eight data bits and one stop bit.&nbsp; The host 
changes the Data line only when the Clock line is low, and data is read by the 
device when Clock is high.&nbsp; This is opposite of what occours in 
device-to-host communication. 
<P>After the stop bit is received, the device will acknowledge the received byte 
by bringing the Data line low and generating one last clock pulse.&nbsp; If the 
host does not release the Data line after the 11th clock pulse, the device will 
continue to generate clock pulses until the the Data line is released (the 
device will then generate an error.) </P>
<P>The host may abort transmission at time before the 11th clock pulse 
(acknowledge bit) by holding Clock low for at least 100 microseconds. </P>
<P>To make this process a little easier to understand, here's the steps the host 
must follow to send data to a PS/2 device: </P>
<BLOCKQUOTE>1)&nbsp;&nbsp; Bring the Clock line low for at least 100 
  microseconds. <BR>2)&nbsp;&nbsp; Bring the Data line low. <BR>3)&nbsp;&nbsp; 
  Release the Clock line. <BR>4)&nbsp;&nbsp; Wait for the device to bring the 
  Clock line low. <BR>5)&nbsp;&nbsp; Set/reset the Data line to send the first 
  data bit <BR>6)&nbsp;&nbsp; Wait for the device to bring Clock high. 
  <BR>7)&nbsp;&nbsp; Wait for the device to bring Clock low. <BR>8)&nbsp;&nbsp; 
  Repeat steps 5-7 for the other seven data bits and the parity bit 
  <BR>9)&nbsp;&nbsp; Release the Data line. <BR>10) Wait for the device to bring 
  Data low. <BR>11) Wait for the device to bring Clock&nbsp; low. <BR>12) Wait 
  for the device to release Data and Clock</BLOCKQUOTE>
<P><BR><FONT color=#000000>Figure 3 shows this graphically and Figure 4 
separates the timing to show which signals are generated by the host, and which 
are generated by the PS/2 device.&nbsp; Notice the change in timing for the 
"ack" bit--the data transition occours when the Clock line is high (rather than 
when it is low as is the case for the other 11 bits.)</FONT> </P>
<P><FONT color=#ff0000>Figure 3:&nbsp; Host-to-Device Communication.</FONT> 
<BR><IMG height=131 alt="" 
src="The PS-2 Mouse-Keyboard Protocol.files/waveform2.jpg" width=504> </P>
<P><FONT color=#ff0000>Figure 4:&nbsp; Detailed host-to-device 
communication.</FONT> <BR><IMG height=247 alt="" 
src="The PS-2 Mouse-Keyboard Protocol.files/waveform3.jpg" width=552> <BR>&nbsp; 
</P>
<P>Referring to Figure 4, there's two time quantities the host looks for. 
&nbsp;(a) is the time it takes the device to begin generating clock pulses after 
the host initially takes the Clock line low, which must be no greater than 15 
ms. (b) is the time it takes for the&nbsp; packet to be sent, which must be no 
greater than 2ms.&nbsp; If either of these time limits is not met, the host 
should generate an error.&nbsp; Immediately after the "ack" is received, the 
host may bring the Clock line low to inhibit communication while it processes 
data.&nbsp; If the command sent by the host requires a response, that response 
must be received no later than 20 ms after the host releases the Clock 
line.&nbsp; If this does not happen, the host generates an 
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