HCS300 datasheet(3/17 Pages) MICROCHIP

3 / 17 page

ã 1997 Microchip Technology Inc.

DS00665A-page 3

AN665

FIGURE 4: HCS300 BLOCK DIAGRAM

The IBM PC-AT® Keyboard Protocol

IBM® was the ﬁrst to introduce the synchronous serial

protocol most of today’s PC-ATs use to communicate

with a keyboard. This now-standard, 5-pole shielding

connector (Figure 5) carries the clock line, data line,

ground, and +5V power supply in order to transmit data

bidirectionally from the keyboard to the PC.

Typically, data travelling from the keyboard to the PC is

accomplished by either key pressure or release infor-

mation. However, some conﬁguration data (i.e., repeat,

delay, and rate) can ﬂow in the opposite direction – for

example, during a system boot. The keyboard drives

the clock line by using open collector drivers. To disable

the keyboard, the PC can keep the clock line low. If the

data line is held low by the PC while the clock line is

high, the computer transmits a requests to send, and

the keyboard goes into receive mode. The keyboard is

only allowed to send data when both the clock line and

data line are high.

FIGURE 5: STANDARD 5-POLE CONNECTOR

FIGURE 6: AT® KEYBOARD PROTOCOL

VSS

VDD

Oscillator

Reset circuit

LED driver

Controller

Power

latching

and

switching

Button input port

32-bit shift register

Encoder

EEPROM

PWM

LED

S3 S2 S1 S0

1 = Clock

2 = Data

3 = GND

4 = GND

5 = +5V

2345

6789

Keyboard

Clock

Keyboard

Data

Start

Bit

Start

Bit

LSB

MSB

Parity

Stop

Bit

MSB

Parity

Stop

Bit

Keyboard pulls low

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