IR Remote with STM32

Today in this tutorial I am going to interface IR Remote with STM32 microcontroller. IR Remote Controllers and receivers follow some standard protocols for sending and receiving data. Some of these protocols are NEC , JVC , SIRC etc. Here we will be discussing only the NEC protocol. I will also use LCD16x2 to display the key pressed on the remote. But first let’s see what NEC protocol is.

What is NEC Protocol

In NEC Protocol, a logic 0 is indicated by 562.5µs pulse burst followed by a 562.5µs space, with a total transmit time of 1.125ms and a logic 1 by 562.5µs pulse burst followed by a 1.6875ms space, with a total transmit time of 2.25ms.

Whenever a key is pressed, the following is transmitted:-
1. A 9 ms leading pulse
2. A 4.5 ms space
3. The 8-bit address
4. The 8-bit logical inverse of the address
5. The 8-bit command
6. The 8-bit logical inverse of the command
7. Finally a 562.5µs pulse burst to show end of message transmission

How to Decode

Decoding any IR protocol is not difficult at all. The IR receiver is a photodiode and pre-amplifier that converts the IR light into an electrical signal. It converts the active HIGH signal to active LOW. Also the signal pulse are continuous as shown in the picture below

Ok so in order to decode first we have to wait for the input pin to go low. Once the pin is LOW, we will wait for the pin to go HIGH. This should take around 9ms.

while (HAL_GPIO_ReadPin (GPIOA, GPIO_PIN_1));   // wait for the pin to go low
while (!(HAL_GPIO_ReadPin (GPIOA, GPIO_PIN_1)));  // wait for the pin to go high.. 9ms LOW

Again wait for the pin to go low, which should take around 4.5 ms. And this will conclude the Start of Frame.

while (HAL_GPIO_ReadPin (GPIOA, GPIO_PIN_1));   // wait for the pin to go low

Now we are going to read the 32 bits and store them in a variable. To read a bit, we are going to check the length of the SPACE followed by the 562.5µs. If this length is more than 1 ms, that means the bit is ‘1’. If the length is less than 1 ms the bit is ‘0’.

for (int i=0; i<32; i++)
{
  count=0;
  while (!(HAL_GPIO_ReadPin (GPIOA, GPIO_PIN_1))); // wait for pin to go high.. this is 562.5us LOW

  while ((HAL_GPIO_ReadPin (GPIOA, GPIO_PIN_1)))  // count the space length while the pin is high
 {
  count++;
  DWT_Delay_us(100);
 }

 if (count > 12) // if the space is more than 1.2 ms
 {
  data |= (1UL << (31-i));   // write 1
 }

 else data &= ~(1UL << (31-i));  // write 0
}

Next we need to assign the values to the key pressed on the Remote. I will also print these characters on the LCD. Below is the code to do that.

switch (code)
{
	case (0xFFA25D):
		lcd_send_cmd (0x86);
		lcd_send_data ('1');
		break;
	
	case (0xFF629D):
		lcd_send_cmd (0x86);
		lcd_send_data ('2');
		break;			

	case (0xFFE21D):
		lcd_send_cmd (0x86);
		lcd_send_data ('3');
		break;

	case (0xFF22DD):
		lcd_send_cmd (0x86);
		lcd_send_data ('4');
		break;

	case (0xFF02FD):
		lcd_send_cmd (0x86);
		lcd_send_data ('5');
		break;

	case (0xFFC23D):
		lcd_send_cmd (0x86);
		lcd_send_data ('6');
		break;

	case (0xFFE01F):
		lcd_send_cmd (0x86);
		lcd_send_data ('7');
		break;

	case (0xFFA857):
		lcd_send_cmd (0x86);
		lcd_send_data ('8');
		break;

	case (0xFF906F):
		lcd_send_cmd (0x86);
		lcd_send_data ('9');
		break;

	case (0xFFB04F):
		lcd_send_cmd (0x86);
		lcd_send_data ('#');
		break;

	case (0XFF6897):
		lcd_send_cmd (0x86);
		lcd_send_data ('*');
		break;			
						
	case (0xFF9867):
		lcd_send_cmd (0x86);
		lcd_send_data ('0');
		break;

	case (0xFF38C7):
		lcd_send_cmd (0x86);
		lcd_send_data ('K');
		break;

	case (0xFF18E7):
		lcd_send_cmd (0x86);
		lcd_send_data ('^');
		break;
			
	case (0xFF10EF):
		lcd_send_cmd (0x86);
		lcd_send_data ('<');
		break;			

	case (0xFF5AA5):
		lcd_send_cmd (0x86);
		lcd_send_data ('>');
		break;
			
	case (0xFF4AB5):
		lcd_send_cmd (0x86);
		lcd_send_data ('u');
		break;
			
	default :
		break;
}