VIA Technologies, Inc.

VT82C42

-2-

The host can program the output port (P20-P23 in AT mode, or P20-P21 in PS/2 mode) or in-out port (P10-

P15 in AT mode, or P12-P15 in PS/2 mode) by issuing a command to the command register on the VT82C42.

The controller will then quickly execute the specified command. Note that P16-P17 is implemented as an

input port only. The host can also transmit data to the keyboard and mouse by issuing a command to the data

register. The data coming to the data register (with A0 = 0, CS# = 0, RD# = 1, and WR# = 0) will be sent to

the keyboard via the keyboard serial interfaces. The data sent to the mouse will be completed by 1) issuing a

D4 command to the command register, 2) then writing the following data byte to the data register (to be sent to

the mouse via mouse serial interface). In either case, the VT82C42 will wait for an acknowledgement from the

keyboard or mouse to complete a transmission. At the same time as the completion of the transmission, the

VT82C42 will raise P24 or P25 (when sending data to mouse) to signal the host of a completion of

transmission. When the controller receives or transmits, the controller does a parity and time-out check. If any

error occurs in the interface or inside the external devices (keyboard or mouse), the controller will reflect that

error in the following status register.

command

decoder

+

scan

mapping

datainput buffer

commandregister/

dataregister

dataoutput buffer

status register

arbitration&

central control

unit

timer

X1, X2

A0

CS

D[7:0]

receivingunit

transmittingunit

clocking

(8 Mhz)

In/Out

port buffer

KBCK

KBDT

MSCK

MSDT

P[17:10]

P[27:20]

RESET

IOR

IOW

mode

selector

T1

T0

Fig 1. Block Diagram for VT82C42