PS/2 Library

The mikroC PRO for PIC provides a library for communication with the common PS/2 keyboard.

  Important :

External dependencies of PS/2 Library

The following variables must be defined in all projects using PS/2 Library: Description : Example :
extern sfr sbit PS2_Data; PS/2 Data line. sbit PS2_Data at RC0_bit;
extern sfr sbit PS2_Clock; PS/2 Clock line. sbit PS2_Clock at RC1_bit;
extern sfr sbit PS2_Data_Direction; Direction of the PS/2 Data pin. sbit PS2_Data_Direction at TRISC0_bit;
extern sfr sbit PS2_Clock_Direction; Direction of the PS/2 Clock pin. sbit PS2_Clock_Direction at TRISC1_bit;

Library Routines



void Ps2_Config();




Initializes the MCU for work with the PS/2 keyboard.


Global variables :

  • PS2_Data: Data signal line
  • PS2_Clock: Clock signal line in
  • PS2_Data_Direction: Direction of the Data pin
  • PS2_Clock_Direction: Direction of the Clock pin
must be defined before using this function.

sbit PS2_Data at RC0_bit;
sbit PS2_Clock at RC1_bit;
sbit PS2_Data_Direction at TRISC0_bit;
sbit PS2_Clock_Direction at TRISC1_bit;
Ps2_Config();         // Init PS/2 Keyboard



unsigned short Ps2_Key_Read(unsigned short *value, unsigned short *special, unsigned short *pressed);


  • 1 if reading of a key from the keyboard was successful
  • 0 if no key was pressed


The function retrieves information on key pressed.

Parameters :

  • value: holds the value of the key pressed. For characters, numerals, punctuation marks, and space value will store the appropriate ASCII code. Routine “recognizes” the function of Shift and Caps Lock, and behaves appropriately. For special function keys see Special Function Keys Table.
  • special: is a flag for special function keys (F1, Enter, Esc, etc). If key pressed is one of these, special will be set to 1, otherwise 0.
  • pressed: is set to 1 if the key is pressed, and 0 if it is released.


PS/2 keyboard needs to be initialized. See Ps2_Config routine.

unsigned short keydata = 0, special = 0, down = 0;
// Press Enter to continue:
do {
  if (Ps2_Key_Read(&keydata, &special, &down)) {
    if (down && (keydata == 16)) break;
} while (1);

Special Function Keys

Key Value returned
F1 1
F2 2
F3 3
F4 4
F5 5
F6 6
F7 7
F8 8
F9 9
F10 10
F11 11
F12 12
Enter 13
Page Up 14
Page Down 15
Backspace 16
Insert 17
Delete 18
Windows 19
Ctrl 20
Shift 21
Alt 22
Print Screen 23
Pause 24
Caps Lock 25
End 26
Home 27
Scroll Lock 28
Num Lock 29
Left Arrow 30
Right Arrow 31
Up Arrow 32
Down Arrow 33
Escape 34
Tab 35

Library Example

This simple example reads values of the pressed keys on the PS/2 keyboard and sends them via UART.

Copy Code To ClipboardCopy Code To Clipboard
unsigned short keydata = 0, special = 0, down = 0;
sbit PS2_Data            at RC0_bit;
sbit PS2_Clock           at RC1_bit;
sbit PS2_Data_Direction  at TRISC0_bit;
sbit PS2_Clock_Direction at TRISC1_bit;

void main() {

  ANSEL  = 0;                                         // Configure AN pins as digital I/O
  ANSELH = 0;
  C1ON_bit = 0;                                       // Disable comparators
  C2ON_bit = 0;
  UART1_Init(19200);                                  // Initialize UART module at 19200 bps
  Ps2_Config();                                       // Init PS/2 Keyboard
  Delay_ms(100);                                      // Wait for keyboard to finish
  UART1_Write(10);                                    // Line Feed
  UART1_Write(13);                                    // Carriage return

  do {
    if (Ps2_Key_Read(&keydata, &special, &down)) {
      if (down && (keydata == 16)) {                  // Backspace
      else if (down && (keydata == 13)) {             // Enter
        UART1_Write('r');                            // send carriage return to usart terminal
        //Usart_Write('n');                          // uncomment this line if usart terminal also expects line feed
                                                      // for new line transition
      else if (down && !special && keydata) {
    Delay_ms(1);                                      // debounce
  } while (1);

HW Connection

Example PS2 keyboard connection

Example of PS2 keyboard connection

Copyright (c) 2002-2012 mikroElektronika. All rights reserved.
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