How to connect a USB keyboard to Arduino

 Have you ever thought about using an Arduino USB keyboard in one of your projects? In fact, this is easier to accomplish than you might think. USB keyboards still support the legacy PS / 2 serial protocol. Therefore, they can easily be used in Arduino projects. The schematic and code in this project should work with all Arduino based Atmega AVR microcontrollers such as Uno, Nano, Mega, etc.

It is rather an example of how to integrate USB or PS / 2 keyboards with Arduino in a simple and economical way. Therefore, in this case, there is no support for modification keys such as Shift or Alt. However, here we are using a fully functional PS / 2 library available through the Arduino IDE library manager.

The components of the project for the interaction of a USB keyboard with Arduino with data output to the LCD display are connected according to the following diagram.

On a breadboard, it might look like this.

The project code is presented below.

#define CLOCK 6 //D-
#define DATA 7  //D+

#include <LiquidCrystal.h>

const char keymap[] = {
  0, 0,  0,  0,  0,  0,  0,  0,
  0, 0,  0,  0,  0,  0, '`', 0,
  0, 0 , 0 , 0,  0, 'q','1', 0,
  0, 0, 'z','s','a','w','2', 0,
  0,'c','x','d','e','4','3', 0,
  0,' ','v','f','t','r','5', 0,
  0,'n','b','h','g','y','6', 0,
  0, 0, 'm','j','u','7','8', 0,
  0,',','k','i','o','0','9', 0,
  0,'.','/','l',';','p','-', 0,
  0, 0,'\'', 0,'[', '=', 0, 0,
  0, 0,13, ']', 0, '\\', 0, 0,
  0, 0, 0, 0, 0, 0, 127, 0,
  0,'1', 0,'4','7', 0, 0, 0,
  '0','.','2','5','6','8', 0, 0,
  0,'+','3','-','*','9', 0, 0,
  0, 0, 0, 0 };

const int rs = 12, en = 11, d4 = 5, d5 = 4, d6 = 3, d7 = 2;
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);

void setup()
{
  Serial.begin(115200);
  pinMode(CLOCK, INPUT_PULLUP); 
  pinMode(DATA, INPUT_PULLUP);
  pinMode(13, OUTPUT);
  lcd.begin(20, 4);
  lcd.cursor();
  lcd.blink();
  bitSet(PCICR, PCIE2); 
  bitSet(PCMSK2, CLOCK);
 }

uint8_t lastscan;
uint8_t line = 0, col = 0;


ISR(PCINT2_vect)
{
  uint16_t scanval = 0;
  for(int i = 0; i<11; i++)
  {
    while(digitalRead(CLOCK));
    scanval |= digitalRead(DATA) << i;
    while(!digitalRead(CLOCK));
  }
  scanval >>= 1;
  scanval &= 0xFF;
  Serial.println(scanval, HEX);
  if(lastscan != 0xF0 && scanval != 0xF0)
  switch(scanval)
  {
    case 0x5A: //Enter
      lcd.setCursor(0, ++line & 0x03);
      col = 0;
      break;
    case 0x66: //Backspace
      lcd.setCursor(--col, line);
      lcd.write(' ');
      lcd.setCursor(col, line);
    break;
    default:
      lcd.write(keymap[scanval]);
      col++;
  }
  lastscan = scanval;
  bitSet(PCIFR, PCIF2);
}

void loop()
{
  /*digitalWrite(13, LOW);
  delay(500);  
  digitalWrite(13, HIGH);
  delay(500);*/
}