Technical Reference Guide

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.

Table 5-13 lists and describes commands that can be issued by the 8042 to the keyboard. Table 5–13. 8042-To-Keyboard Commands

 

Table 5-13.

 

 

8042-To-Keyboard Commands

 

 

Command

Value

Description

 

 

Set/Reset Status Indicators

EDh

Enables LED indicators. Value EDh is followed by an

 

 

option byte that specifies the indicator as follows:

 

 

Bits <7..3> not used

 

 

 

 

Bit <2>, Caps Lock (0 = off,

1 = on)

 

 

Bit <1>, NUM Lock (0 = off,

1 = on)

 

 

Bit <0>, Scroll Lock (0 = off,

1 = on)

Echo

EEh

Keyboard returns EEh when previously enabled.

Invalid Command

EFh/F1h

These commands are not acknowledged.

Select Alternate Scan Codes

F0h

Instructs the keyboard to select another set of scan codes

 

 

and sends an option byte after ACK is received:

 

 

01h = Mode 1

 

 

 

 

02h = Mode 2

 

 

 

 

03h = Mode 3

 

 

Read ID

F2h

Instructs the keyboard to stop scanning and return two

 

 

keyboard ID bytes.

 

 

Set Typematic Rate/Display

F3h

Instructs the keyboard to change typematic rate and delay

 

 

to specified values:

 

 

 

 

Bit <7>, Reserved - 0

 

 

 

 

Bits <6,5>, Delay Time

 

 

 

00 = 250 ms

 

 

 

01 = 500 ms

 

 

 

10 = 750 ms

 

 

 

11 = 1000 ms

 

 

 

Bits <4..0>, Transmission Rate:

 

 

00000 = 30.0 ms

 

 

00001 = 26.6 ms

 

 

00010 = 24.0 ms

 

 

00011 = 21.8 ms

 

 

:

 

 

 

 

11111 = 2.0 ms

 

Enable

F4h

Instructs keyboard to clear output buffer and last typematic

 

 

key and begin key scanning.

 

Default Disable

F5h

Resets keyboard to power-on default state and halts

 

 

scanning pending next 8042 command.

Set Default

F6h

Resets keyboard to power-on default state and enable

 

 

scanning.

 

 

Set Keys - Typematic

F7h

Clears keyboard buffer

and sets default scan code set. [1]

Set Keys - Make/Brake

F8h

Clears keyboard buffer

and sets default scan code set. [1]

Set Keys - Make

F9h

Clears keyboard buffer

and sets default scan code set. [1]

Set Keys - Typematic/Make/Brake

FAh

Clears keyboard buffer

and sets default scan code set. [1]

Set Type Key - Typematic

FBh

Clears keyboard buffer and prepares to receive key ID. [1]

Set Type Key - Make/Brake

FCh

Clears keyboard buffer and prepares to receive key ID. [1]

Set Type Key - Make

FDh

Clears keyboard buffer and prepares to receive key ID. [1]

Resend

FEh

8042 detected error in keyboard transmission.

Reset

FFh

Resets program, runs keyboard BAT, defaults to Mode 2.

Note:

[1] Used in Mode 3 only.

Compaq D315 and hp d325 Personal Computers5-17

Featuring the AMD Athlon XP Processor

Second Edition - April 2003