Keyboard/Pointing Device Interface; Keyboard Interface Operation - HP Compaq dc7900 USDT Technical Reference Manual
Compaq dc7900 series business desktop computers
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5.6 Keyboard/Pointing Device Interface
The keyboard/pointing device interface function is provided by the SIO controller component,
which integrates 8042-compatible keyboard controller logic (hereafter referred to as simply the
"8042") to communicate with the keyboard and pointing device using bi-directional serial data
transfers. The 8042 handles scan code translation and password lock protection for the keyboard
as well as communications with the pointing device.
5.6.1 Keyboard Interface Operation
The data/clock link between the 8042 and the keyboard is uni-directional for Keyboard Mode 1
and bi-directional for Keyboard Modes 2 and 3. (These modes are discussed in detail in
Appendix C). This section describes Mode 2 (the default) mode of operation.
Communication between the keyboard and the 8042 consists of commands (originated by either
the keyboard or the 8042) and scan codes from the keyboard. A command can request an action
or indicate status. The keyboard interface uses IRQ1 to get the attention of the CPU.
The 8042 can send a command to the keyboard at any time. When the 8042 wants to send a
command, the 8042 clamps the clock signal from the keyboard for a minimum of 60 us. If the
keyboard is transmitting data at that time, the transmission is allowed to finish. When the 8042 is
ready to transmit to the keyboard, the 8042 pulls the data line low, causing the keyboard to
respond by pulling the clock line low as well, allowing the start bit to be clocked out of the 8042.
The data is then transferred serially, LSb first, to the keyboard (Figure 5-6). An odd parity bit is
sent following the eighth data bit. After the parity bit is received, the keyboard pulls the data line
low and clocks this condition to the 8042. When the keyboard receives the stop bit, the clock line
is pulled low to inhibit the keyboard and allow it to process the data.
Start
Data
Clock
Figure 5-6. 8042-To-Keyboard Transmission of Code EDh, Timing Diagram
Control of the data and clock signals is shared by the 8042 and the keyboard depending on the
originator of the transferred data. Note that the clock signal is always generated by the keyboard.
After the keyboard receives a command from the 8042, the keyboard returns an ACK code. If a
parity error or timeout occurs, a Resend command is sent to the 8042.
Technical Reference Guide
D1
D2
D0
Bit
(LSb)
0
1
0
1
Tcy
Tcl Tch
Th
Parameter
Tcy (Cycle Time)
Tcl (Clock Low)
Tch (Clock High)
Th (Data Hold)
Tss (Stop Bit Setup)
Tsh (Stop Bit Hold)
D3
D4
D5
1
0
1
Minimum
Maximum
0 us
80 us
25 us
35 us
25 us
45 us
0 us
25 us
8 us
20 us
15 us
25 us
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Input/Output Interfaces
D6
Parity
D7
Stop
(MSb)
Bit
1
1
1
0
Tss
Tsh
5-9
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