STM32 study notes: infrared remote control

1. Brief Introduction

Infrared remote control is a wireless, non-contact control technology with strong anti-interference ability, reliable information transmission, low power consumption and low cost.

The encoding methods for infrared remote control that are currently widely used are: NEC protocol for PWM (pulse width modulation) and RC-5 protocol for Philips PPM (pulse position modulation).

1.1 NEC Protocol Definition

The bit definition of NEC code: One pulse corresponds to a continuous carrier of 560us. It takes 2.25ms (560us pulse + 1680us low level) to transmit a logic 1, and 1.125ms (560us pulse + 560us low level) to transmit a logic 0. The remote control receiver is at a low level when it receives a pulse, and at a high level when there is no pulse. In this way, the signal we receive at the receiving head is: logic 1 should be 560us low + 1680us high, and logic 0 should be 560us low + 560us high.

Transmitter logic:

Remote control receiving head logic:

1.2 NEC Protocol Features

(1) 8-bit address and 8-bit instruction length;

(2) Address and command are transmitted twice (to ensure reliability);

(3) PWM pulse position modulation, using the duty cycle of the transmitted infrared carrier to represent "0" and "1";

(4) The carrier frequency is 38Khz;

(5) The bit time is 1.125ms or 2.25ms;

1.3 Data format of NEC remote control commands

The reverse code is used to increase the reliability of transmission. The continuous code specified by the NEC code (composed of 9ms low level + 2.5m high level + 0.56ms low level + 97.94ms high level) will send a repeated code, i.e. a continuous code, if the key is not released after a frame of data is sent.

2. Software Implementation

Above we have a basic understanding of the NEC format of infrared code sending and receiving, and we can complete the corresponding program according to the communication protocol.

Program logic:

2.1 Initialization

void Remote_Init(void)

{

GPIO_InitTypeDef GPIO_InitStructure;

NVIC_InitTypeDef NVIC_InitStructure;

TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;

TIM_ICInitTypeDef TIM_ICInitStructure;

RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA,ENABLE);

RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM5,ENABLE);

GPIO_InitStructure.GPIO_Pin = GPIO_Pin_1;

GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IPD; //Pull-down input

GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;

GPIO_Init(GPIOA,&GPIO_InitStructure);

GPIO_SetBits(GPIOA,GPIO_Pin_1);

TIM_TimeBaseStructure.TIM_Period = 10000; //Set the automatic reload value, 10ms overflow

TIM_TimeBaseStructure.TIM_Prescaler = (72-1); //Prescaler, 1MHz counting frequency, 1us plus one

TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1; //Clock division

TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up; //Upward counting mode

TIM_TimeBaseInit(TIM5,&TIM_TimeBaseStructure);

TIM_ICInitStructure.TIM_Channel = TIM_Channel_2; //IC2 is mapped to TI5

TIM_ICInitStructure.TIM_ICPolarity = TIM_ICPolarity_Rising; //Rising edge capture

TIM_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI;

TIM_ICInitStructure.TIM_ICPrescaler = TIM_ICPSC_DIV1; //No frequency division

TIM_ICInitStructure.TIM_ICFilter = 0x03; //IC4F = 0011, input filter 8 timer clock cycle filtering

TIM_ICInit(TIM5,&TIM_ICInitStructure); //Initialize the timer input capture channel

TIM_Cmd(TIM5,ENABLE); //Enable timer 5

NVIC_InitStructure.NVIC_IRQChannel = TIM5_IRQn;//TIM5中断

NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 1; //Preemption priority 1

NVIC_InitStructure.NVIC_IRQChannelSubPriority = 3; //From priority 3

NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;

NVIC_Init(&NVIC_InitStructure);

TIM_ITConfig(TIM5,TIM_IT_Update|TIM_IT_CC2,ENABLE); //Enable update interrupt, allow CC2IE to capture interrupt

}

2.2 Interrupt capture

u8 RmtSta=0;

u16 Dval;

u32 RmtRec=0;

u8 RmtCnt=0;

void TIM5_IRQHandler(void)

{

if(TIM_GetITStatus(TIM5,TIM_IT_Update)!= RESET)

{

if(RmtSta&0x80) //Data received flag

{

RmtSta &= ~0x10; //Cancel rising edge capture mark

if((RmtSta&0x0F)==0x00)

RmtSta |= 1<<6;

if((RmtSta&0x0F)<14)

RmtSta++;

else

{

RmtSta &= ~(1<<7); // Clear the boot flag

RmtSta &= 0xF0; // Clear the counter

}

}

}

if(TIM_GetITStatus(TIM5,TIM_IT_CC2)!=RESET)

{

if(RDATA) // rising edge has been captured

{

TIM_OC2PolarityConfig(TIM5,TIM_ICPolarity_Falling); //CC1P=1, set to falling edge capture

TIM_SetCounter(TIM5,0); //Clear the timer value

RmtSta |= 0x10; //Mark rising edge has been captured

}

else

{

Dval = TIM_GetCapture2(TIM5); //Read the value of CCR1

TIM_OC2PolarityConfig(TIM5,TIM_ICPolarity_Rising); //Set to rising edge capture

if(RmtSta&0x10)

{

if(RmtSta&0x80) //Receive the boot code

{

if(Dval>300&&Dval<800)//high level is 560us

{

RmtRec <<= 1;

RmtRec |= 0; //Received 0 code

}

else if(Dval>1400&&Dval<1800)//High level is 1680us

{

RmtRec <<= 1;

RmtRec |= 1; //Receive 1 code

}

else if(Dval>2200&&Dval<2600)//Continuous code judgment

{

RmtCnt++;

RmtSta &= 0xF0; // Clear the counter

}

}

else if(Dval>4200&&Dval<4700)

{

RmtSta |= 1<<7; //Record the received boot code

RmtCnt = 0;

}

}

RmtSta &= ~(1<<4);

}

}

TIM_ClearFlag(TIM5,TIM_IT_Update|TIM_IT_CC2);

}

2.3 Remote Control Key Value Scanning

u8 Remote_Scan(void)

{

u8 sta=0;

u8 t1,t2;

if(RmtSta&(1<<6))//Get the information of a key

{

t1 = RmtRec>>24; //Address code

t2 = (RmtRec>>16) & 0xFF; //Address inverse code

if((t1==(u8)~t2)&&t1==REMOTE_ID)//Check remote control identification code and remote control receiving address

{

t1 = RmtRec >> 8; //Control code

t2 = RmtRec; //control inverse code

if(t1==(u8)~t2)

sta = t1;

}

if((sta==0)||((RmtSta&0x80)==0))//Receive error or key not pressed

{

RmtSta &= ~(1<<6); // Clear the valid flag of receiving key

RmtCnt = 0;

}

}

return sta;

}