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Chapter 6: PS/2 Mouse/Keyboard Port
bidirectional keyboard communications. As shown in Figure 6-2 , the attached keyboard or
mouse writes a bit on the data line when the clock signal is High, and the host reads the
data line when the clock signal is Low.
Table 6-2:
PS/2 Bus Timing
Symbol
T CK
T SU
T HLD
Parameter
Clock High or Low time
Data-to-clock setup time
Clock-to-data hold time
Edge 0
T CK T CK
Min
30 μ s
5 μ s
5 μ s
Edge 10
Max
50 μ s
25 μ s
25 μ s
CLK (PS2C)
DATA (PS2D)
T SU
T HLD
'0' start bit
'1' stop bit
UG130_c6_02_042404
Figure 6-2: PS/2 Bus Timing Waveforms
The following site contains additional information on the PS/2 bus protocol:
?
PS/2 Mouse/Keyboard Protocol
http://www.computer-engineering.org/ps2protocol/
Keyboard
The keyboard uses open-collector drivers so that either the keyboard or the host can drive
the two-wire bus. If the host never sends data to the keyboard, then the host can use simple
input pins.
A PS/2-style keyboard uses scan codes to communicate key press data. Nearly all
keyboards in use today are PS/2 style. Each key has a single, unique scan code that is sent
whenever the corresponding key is pressed. The scan codes for most keys appear in
Figure 6-3 .
If the key is pressed and held, the keyboard repeatedly sends the scan code every 100 ms or
so. When a key is released, the keyboard sends a “F0” key-up code, followed by the scan
code of the released key. The keyboard sends the same scan code, regardless if a key has
different “shift” and “non-shift” characters and regardless whether the Shift key is pressed
or not. The host determines which character is intended.
Some keys, called extended keys, send an “E0” ahead of the scan code and furthermore,
they may send more than one scan code. When an extended key is released, a “E0 F0” key-
up code is sent, followed by the scan code.
28
www.xilinx.com
Spartan-3 FPGA Starter Kit Board User Guide
UG130 (v1.2) June 20, 2008
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