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.
8-bit
Microcontroller
Application
Note
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.
Vcc
AVR
VCC
+5V
INT0
(or INT1)
Clock
Pxy
Data
GND
Keyboard
GND
Rev. 1235B–AVR–05/02
1
Table 1. AT Keyboard Connector Pin Assignments
4
1
2
5
3
6
4
5
3
2
1
AT Computer
Timing
Signals
DIN41524, Female at
Computer, 5-pin DIN 180o
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
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 keyboard 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
The AT keyboard has a scan code associated with each key. When a key is pressed,
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 transmitted 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
two to five bytes. The first byte is always $E0. This is also true for the “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 characters 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:
2
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.
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The Algorithm
Keyboard reception is handled by the interrupt function INT0_interrupt. The reception
will operate independent of the rest of the program.
The algorithm is quite simple: Store the value of the data line at the leading edge of the
clock pulse. This is easily handled if the clock line is connected to the INT0 or INT1 pin.
The interrupt function will be executed at every edge of the clock cycle, and data will be
stored at the falling edge. After all bits are received, the data can be decoded. This is
done by calling the decode function. For character keys, this function will store an ASCII
character in a buffer. It will take into account if the shift key is held down when a key is
pressed. Other keys like function keys, navigation keys (arrow keys, page up/down keys
etc.) and modifier keys like Ctrl and Alt are ignored.
The mapping from scan codes to ascii characters are handled with table look-ups, one
table for shifted characters and one for un-shifted.
Modifications and
Improvements
If the host falls out of sync with the keyboard, all subsequent data received will be
wrong. One way to solve this is to use a time out. If 11 bits are not received within
1.5 ms, some error have occurred. The bit counter should be reset and the faulty data
discarded.
If keyboard parameters like typematic rate and delay are to be set, data must be sent to
the keyboard. This can be done as described earlier. For the commands, see the keyboard manufacturer’s specifications.
Figure 2. Timing for Keyboard to Host Transfer
Clock
Data
Main.c
Start
Bit 0
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
Parity
Stop
#include <pgmspace.h>
#include <stdio.h>
#include <stdlib.h>
#include "io8515.h"
#include "serial.h"
#include "gpr.h"
#include "kb.h"
void main(void)
{
unsigned char key;
init_uart();
// Initializes the UART transmit buffer
init_kb();
// Initialize keyboard reception
while(1)
{
key=getchar();
putchar(key);
delay(100);
}
}
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Low_level_init.c
#include <ina90.h>
#include <io8515.h>
int __low_level_init(void)
{
UBRR = 12;
// 19200bps @ 4 MHz
UCR
// TX enable
= 0x08;
GIMSK= 0x40;
// Enable INT0 interrupt
_SEI();
return 1;
}
Serial.c
#include <stdio.h>
#include <pgmspace.h>
#include <io8515.h>
/* SFR declarations */
#include "serial.h"
#define ESC 0x1b
#define BUFF_SIZE 64
flash char CLR[] = {ESC, ’[’,’H’, ESC, ’[’, ’2’, ’J’,0};
unsigned char UART_buffer[BUFF_SIZE];
unsigned char *inptr, *outptr;
unsigned char buff_cnt;
void init_uart(void)
{
inptr =
UART_buffer;
outptr = UART_buffer;
buff_cnt = 0;
}
void clr(void)
{
puts_P(CLR);
VT100 terminal
// Send a ’clear screen’ to a
}
int putchar(int c)
{
if (buff_cnt<BUFF_SIZE)
{
*inptr = c;
// Put character into buffer
inptr++;
// Increment pointer
buff_cnt++;
if (inptr >= UART_buffer + BUFF_SIZE)
4
// Pointer wrapping
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inptr = UART_buffer;
UCR = 0x28;
// Enable UART Data register
// empty interrupt
return 1;
} else {
return 0;
// Buffer is full
}
}
// Interrupt driven transmitter
interrupt [UART_UDRE_vect] void UART_UDRE_interrupt(void)
{
UDR = *outptr;
// Send next byte
outptr++;
// Increment pointer
if (outptr >= UART_buffer + BUFF_SIZE)
// Pointer wrapping
outptr = UART_buffer;
if(--buff_cnt == 0)
// If buffer is empty:
UCR = UCR && (1<<UDRIE);
//
disabled interrupt
}
Kb.c
#include <pgmspace.h>
#include "kb.h"
#include "serial.h"
#include "gpr.h"
#include "scancodes.h"
#define BUFF_SIZE 64
unsigned char edge, bitcount;// 0 = neg.
1 = pos.
unsigned char kb_buffer[BUFF_SIZE];
unsigned char *inpt, *outpt;
unsigned char buffcnt;
void init_kb(void)
{
inpt =
kb_buffer;// Initialize buffer
outpt = kb_buffer;
buffcnt = 0;
MCUCR = 2; // INT0 interrupt on falling edge
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edge = 0;
// 0 = falling edge
1 = rising edge
bitcount = 11;
}
interrupt [INT0_vect] void INT0_interrupt(void)
{
static unsigned char data;// Holds the received scan code
if (!edge) // Routine entered at falling edge
{
if(bitcount < 11 && bitcount > 2)// Bit 3 to 10 is data. Parity bit,
{
// start and stop bits are ignored.
data = (data >> 1);
if(PIND & 8)
data = data | 0x80;// Store a ’1’
}
MCUCR = 3;// Set interrupt on rising edge
edge = 1;
} else {
// Routine entered at rising edge
MCUCR = 2;// Set interrupt on falling edge
edge = 0;
if(--bitcount == 0)// All bits received
{
decode(data);
bitcount = 11;
}
}
}
void decode(unsigned char sc)
{
static unsigned char is_up=0, shift = 0, mode = 0;
unsigned char i;
if (!is_up)// Last data received was the up-key identifier
{
switch (sc)
{
case 0xF0 :// The up-key identifier
is_up = 1;
break;
case 0x12 :// Left SHIFT
shift = 1;
break;
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case 0x59 :// Right SHIFT
shift = 1;
break;
case 0x05 :// F1
if(mode == 0)
mode = 1;// Enter scan code mode
if(mode == 2)
mode = 3;// Leave scan code mode
break;
default:
if(mode == 0 || mode == 3)// If ASCII mode
{
if(!shift)// If shift not pressed,
{
// do a table look-up
for(i = 0; unshifted[i][0]!=sc && unshifted[i][0]; i++);
if (unshifted[i][0] == sc) {
put_kbbuff(unshifted[i][1]);
}
} else {// If shift pressed
for(i = 0; shifted[i][0]!=sc && shifted[i][0]; i++);
if (shifted[i][0] == sc) {
put_kbbuff(shifted[i][1]);
}
}
} else{ // Scan code mode
print_hexbyte(sc);// Print scan code
put_kbbuff(’ ’);
put_kbbuff(’ ’);
}
break;
}
} else {
is_up = 0;// Two 0xF0 in a row not allowed
switch (sc)
{
case 0x12 :// Left SHIFT
shift = 0;
break;
case 0x59 :// Right SHIFT
shift = 0;
break;
case 0x05 :// F1
if(mode == 1)
mode = 2;
if(mode == 3)
mode = 0;
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break;
case 0x06 :// F2
clr();
break;
}
}
}
void put_kbbuff(unsigned char c)
{
if (buffcnt<BUFF_SIZE)// If buffer not full
{
*inpt = c;// Put character into buffer
inpt++;
// Increment pointer
buffcnt++;
if (inpt >= kb_buffer + BUFF_SIZE)// Pointer wrapping
inpt = kb_buffer;
}
}
int getchar(void)
{
int byte;
while(buffcnt == 0);// Wait for data
byte = *outpt;// Get byte
outpt++;
// Increment pointer
if (outpt >= kb_buffer + BUFF_SIZE)// Pointer wrapping
outpt = kb_buffer;
buffcnt--; // Decrement buffer count
return byte;
}
8
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Gpr.c
#include "gpr.h"
void print_hexbyte(unsigned char i)
{
unsigned char h, l;
h = i & 0xF0;
// High nibble
h = h>>4;
h = h + ’0’;
if (h > ’9’)
h = h + 7;
l = (i & 0x0F)+’0’;
// Low nibble
if (l > ’9’)
l = l + 7;
putchar(h);
putchar(l);
}
void delay(char d)
{
char i,j,k;
for(i=0; i<d; i++)
for(j=0; j<40; j++)
for(k=0; k<176; k++);
}
Pindefs.h
//*********************
// Pin definition file
//*********************
// Keyboard konnections
#define PIN_KB
PIND
#define PORT_KB PORTD
#define CLOCK
2
#define DATAPIN 3
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Scancodes.h
// Unshifted characters
flash unsigned char unshifted[][2] = {
0x0d,9,
0x0e,’|’,
0x15,’q’,
0x16,’1’,
0x1a,’z’,
0x1b,’s’,
0x1c,’a’,
0x1d,’w’,
0x1e,’2’,
0x21,’c’,
0x22,’x’,
0x23,’d’,
0x24,’e’,
0x25,’4’,
0x26,’3’,
0x29,’ ’,
0x2a,’v’,
0x2b,’f’,
0x2c,’t’,
0x2d,’r’,
0x2e,’5’,
0x31,’n’,
0x32,’b’,
0x33,’h’,
0x34,’g’,
0x35,’y’,
0x36,’6’,
0x39,’,’,
0x3a,’m’,
0x3b,’j’,
0x3c,’u’,
0x3d,’7’,
0x3e,’8’,
0x41,’,’,
0x42,’k’,
0x43,’i’,
0x44,’o’,
0x45,’0’,
0x46,’9’,
0x49,’.’,
0x4a,’-’,
0x4b,’l’,
0x4c,'ø',
0x4d,'p',
0x4e,'+',
0x52,'æ',
0x54,'å',
0x55,'\\',
10
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AVR313
0x5a,13,
0x5b,'¨',
0x5d,'\'',
0x61,'<',
0x66,8,
0x69,'1',
0x6b,'4',
0x6c,'7',
0x70,'0',
0x71,',',
0x72,'2',
0x73,'5',
0x74,'6',
0x75,'8',
0x79,'+',
0x7a,'3',
0x7b,'-',
0x7c,'*',
0x7d,'9',
0,0
};
// Shifted characters
flash unsigned char shifted[][2] = {
0x0d,9,
0x0e,'§',
0x15,'Q',
0x16,'!',
0x1a,'Z',
0x1b,'S',
0x1c,'A',
0x1d,'W',
0x1e,'"',
0x21,'C',
0x22,'X',
0x23,'D',
0x24,'E',
0x25,'¤',
0x26,'#',
0x29,' ',
0x2a,'V',
0x2b,'F',
0x2c,'T',
0x2d,'R',
0x2e,'%',
0x31,'N',
0x32,'B',
0x33,'H',
0x34,'G',
0x35,'Y',
0x36,'&',
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0x39,’L’,
0x3a,’M’,
0x3b,’J’,
0x3c,’U’,
0x3d,’/’,
0x3e,’(’,
0x41,’;’,
0x42,’K’,
0x43,’I’,
0x44,’O’,
0x45,’=’,
0x46,’)’,
0x49,’:’,
0x4a,’_’,
0x4b,’L’,
0x4c,'Ø',
0x4d,'P',
0x4e,'?',
0x52,'Æ',
0x54,'Å',
0x55,'`',
0x5a,13,
0x5b,'^',
0x5d,'*',
0x61,'>',
0x66,8,
0x69,'1',
0x6b,'4',
0x6c,'7',
0x70,'0',
0x71,',',
0x72,'2',
0x73,'5',
0x74,'6',
0x75,'8',
0x79,'+',
0x7a,'3',
0x7b,'-',
0x7c,'*',
0x7d,'9',
0,0
};
12
AVR313
1235B–AVR–05/02
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Printed on recycled paper.
1235B–AVR–05/02
0M