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HT6542B

Keyboard Controller with PS/2

Holtek Semiconductor 

HT6542B

Functional Description

Power on

When power is switched on, the HT6542B auto-

detect the motherboard type(AT or PS/2), then

disable the keyboard/mouse and waits for the

self-test command to perform a self-test. If no

error is detected during self-test, HEX 55 is

registered in the output buffer (note that any

value other than HEX 55 would indicate

HT6542B failure) and the keyboard interface is

enabled. The HT6542B is now ready to receive

a system command or keyboard data.

Keyboard/mouse data transmission

The keyboard/mouse transmits data to the con-

troller in an 11-bit format in sync, with the

keyboard/mouse clock signal. If this transmis-

sion is not completed within the specified pe-

riod, the HT6542B will register HEX FF into

the output buffer and set the "transmit time-

out" error bit in the STATUS REGISTER to 1.

Controller data transmission

The controller transmits data to the key-

board/mouse in the same manner as it receives

data from the keyboard/mouse. When the

HT6542B starts transmitting data and the key-

board/mouse does not start receiving (does not

start clocking) or data transmission is not com-

pleted within 15ms, the HT6542B will register

HEX FE into the output buffer and set the

"transmit time-out" error bit in the STATUS

REGISTER to 1.

Keyboard inhibited (KBD-INH to low)

If the KBD-INH is switched to low, the key-

board/mouse is inhibited. The HT6542B receive

keyboard/mouse code and check the KBD-INH

status, if inhibited the keyboard SCAN CODE

and mouse code will be ignored and the key-

board/mouse command response is registered

into the HT6542B’s output buffer.

Status register notations

The STATUS REGISTER is located in HEX 64

of the I/O. It provides the HT6542B and inter-

face status to the system. The following are the

definitions for each bit:

• b0(OBF): Output buffer full

This bit is set while the HT6542B is sending

data to the output buffer and cleared when the

system reads the output buffer(I/O HEX 60H).

• b1(IBF): Input buffer full

This bit is set while the system is sending

data to the HT6542B’s input buffer and

cleared when the HT6542B reads the input

buffer data.

• b2: System flag

This bit is 0 after power-on reset, set to 1 after

self-test OK.

• b3: Command/Data

When the system writes the data to the

HT6542B from I/O 64H, this bit becomes 1.

Reset to 0 if from I/O 60H.

• b4:

This bit reflects the KBD-INH status when-

ever data is placed in the HT6542B’s output

buffer.

• b5: Auxiliary Output Buffer Full.

0: The HT6542B’s output buffer is a keyboard

data.

1: The HT6542B’s output buffer is a mouse

data.

• b6: Transmit time-out

Set to 1 when the keyboard or mouse is not

able to completely transmite data to the

HT6542B within the specified period.

• b7: Parity error

1: The HT6542B has received the key-

board/mouse code with a parity error.

(should be odd parity).

Output buffer

The output buffer is located in I/O HEX 60. It is

used to transmit keyboard/mouse code or key-

board controller response data. The output buff-

er data is valid only when OBF=1.

Input buffer

The input buffer is located in I/O HEX 60 or

HEX 64. The system writes command and data

into this port in the following categories:

• Data written to I/O HEX 64 as command

write.

• Data written to I/O HEX 60 as data write.

5

30th Nov ’95

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