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AVR313: Interfacing the PC AT Keyboard

Features

• Interfacing Standard PC AT Keyboards

• Requires Only Two I/O Pins. One of them must be an External Interrupt Pin

• No Extra Hardware Required

• Complete Example in C, Implementing a Keyboard to Serial Converter

Introduction

Most microcontrollers requires some kind of a human interface. This application note

describes one way of doing this using a standard PC AT keyboard.

The Physical Interface

The physical interface between the keyboard and the host is shown in Figure 1. Two

signal lines are used, clock and data. The signal lines are open connector, with pullup

resistors located in the keyboard. This allows either the keyboard or the host system

to force a line to low level. Two connector types are available, the 5-pin DIN connector

of “5D” type, and the smaller six-pin mini-DIN. The pin assignments are shown in

Table 1.

Figure 1. The Interface.

Keyboard

+5V

Clock

Data

GND

VCC

GND

INT0

(or INT1)

8-bit

Microcontroller

Application

Note

Rev. 1235B–AVR–05/02

AVR313

1235B–AVR–05/02

Timing The timing for the data transferred from the keyboard to the host is shown in Figure 2.

The protocol is: one start bit (always 0), eight data bits, one odd parity bit and one stop

bit (always 1). The data is valid during the low period of the clock pulse. The keyboard is

generating the clock signal, and the clock pulses are typically 30-50 µs low and 30-50 µs

high.

The host system can send commands to the keyboard by forcing the clock line low. It

then pulls the data line low (the start bit). Now, the clock line must be released. The key-

board will count 10 clock pulses. The data line must be set up to the right level by the

host before the trailing edge of the clock pulse. After the tenth bit, the keyboard checks

for a high level on the data line (the stop bit), and if it is high, it forces it low. This tells the

host that the data is received by the keyboard. The software in this design note will not

send any commands to the keyboard.

Scan Codes TheATkeyboardhasascancodeassociatedwitheachkey.Whenakeyispressed,

this code is transmitted. If a key is held down for a while, it starts repeating. The repeat

rate is typically 10 per second. When a key is released, a “break” code ($F0) is transmit-

ted followed by the key scan code. For most of the keys, the scan code is one byte.

Some keys like the

Home

Insert

and

Delete

keys have an extended scan code, from

twotofivebytes.Thefirstbyteisalways$E0.Thisisalsotrueforthe“break” sequence,

e.g., E0 F0 xx…

AT keyboards are capable of handling three sets of scan codes, where set two is

default. This example will only use set two.

The Software The code supplied with this application note is a simple keyboard to RS-232 interface.

The scan codes received from the keyboard are translated into appropriate ASCII char-

acters and transmitted by the UART. The source code is written in C, and is easily

modified and adaptable to all AVR microconrollers with SRAM.

Note: The linkerfile (AVR313.xcl) included in the software archive has to be included instead of

the standard linkerfile. This is done from the include menu under XLINK – Options. The

linker file applies to AT90S8515 only.

Table 1. AT Keyboard Connector Pin Assignments

AT Computer

Signals

DIN41524, Female at

Computer, 5-pin DIN 180

6-pin Mini DIN PS2 Style

Female at Computer

Clock 1 5

Data 2 1

nc 3 2,6

GND 4 3

+5V 5 4

Shield Shell Shell

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筱菂

2013-06-15

能用，资料不详细
xjoxjo1

2012-07-20

能用，资料不详细
wanffee

2018-12-01

引用的头文件哪里找？

athlan2000

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