Analog-to-digital converter ADC

6.1 ADC structure and register description

The main function of the analog-to-digital converter (ADC) is to convert analog signals into digital signals for data processing by the microcontroller.

ADC is divided into successive comparison type, double integration type and ∑-∆ type according to the conversion principle.

The successive approximation ADC converts analog signals into digital signals through successive comparisons. It has a fast conversion speed but low accuracy and is the most commonly used ADC.

The dual-integral ADC converts analog signals into digital signals by integrating twice. It has high accuracy and strong anti-interference ability, but it is slow. It is mainly used in measuring instruments such as multimeters.

∑-∆ ADC has the dual advantages of successive approximation and dual integration, and is gradually being widely used.

The STM32ADC is a 12-bit successive comparator with up to 18 channels. It can measure 16 external and 2 internal signal sources. The conversion of each channel can be performed in word, continuous, scan or discontinuous mode. The conversion result can be stored in a 16-bit data register in left-aligned or right-aligned mode.

The STM32ADC analog watchdog feature allows the application to detect if the input voltage exceeds the user-defined high/low thresholds

The input clock must not exceed 14MHz and is generated by dividing PCLK2

The STM32ADC is mainly composed of analog multiplexers, analog-to-digital converters, data registers, and trigger selection.

The conversion channels are divided into two groups: regular channels and injection channels.

Regular channels consist of up to 16 channels, converted in sequence

The injection channel consists of up to 4 channels, which can be inserted into the conversion

GPIO pins used by ADC

Channel 16 of ADC1 is internally connected to the temperature sensor

Channel 17 is internally connected to the reference power supply VREFINT

The ADC operates through 20 registers

6.2 ADC Design Example

    6.2.1 Using ADC1 regular channel to realize analog-to-digital conversion of external analog signals

Registers and their contents related to regular channels

ADC regular channel initialization subroutine

//ADC1 initialization subroutine

void Adc1_Init(void)

{

    RCC->APB2ENR |= 1<<3; //Turn on GPIOB clock

    RCC->APB2ENR |= 1<<9; //Turn on ADC1 clock

    GPIOB->CRL &=0xffffff00; //PB.01 (IN9), PB.00 (IN8) analog input

    ADC1->CR1 |= 1<<8; //Scan mode

    ADC1->CR2 |= 1<<20; //Regular channel external trigger: TIM1_CH1

    ADC1->SQR1 |= 1<<20; //Regular channel sequence length: 2

    ADC1->SQR3 |= 9; //1st conversion channel: IN9

    ADC1->SQR3 |= 8<<5; //2nd conversion channel: IN8

    ADC1->CR2 |= 1; //Turn on ADC

    ADC1->CR2 |= 1<<2; //Calibrate ADC

    while(ADC1->CR2 & 1<<2); //Wait for calibration to complete

}

ADC regular channel processing subroutine

//ADC1 processing subroutine

void Adc1_Proc(void)

{

    if(ADC1->SR & 2) //EOC=1 (conversion ends)

    {    

        adc1_dat[adc1_num] = ADC1->DR; //Read conversion value

        if(++adc1_num == 2)

            adc1_num = 0;

    }

}

6.2.2 Using ADC1 injection channel to measure the temperature of the internal temperature sensor

There is a temperature sensor in STM32, which is connected to channel 16 of ADC1 and can be used to measure the temperature of the chip

The maximum sampling time of the temperature sensor is 17.1ms, the temperature range is -40~125°C, and the temperature calculation formula is as follows:

Where: V is the voltage value of the temperature sensor, N is the digital value after analog-to-digital conversion

Injection channel related registers and their contents

ADC injection channel initialization subroutine

//ADC1 initialization subroutine

void Adc1_Init(void)

{

    RCC->APB2ENR |= 1<<3; //Turn on GPIOB clock

    RCC->APB2ENR |= 1<<9; //Turn on ADC1 clock

    GPIOB->CRL &=0xffffff00; //PB.01 (IN9), PB.00 (IN8) analog input

    ADC1->CR1 |= 1<<8; //Scan mode

    ADC1->CR2 |= 1<<20; //Regular channel external trigger: TIM1_CH1

    ADC1->SQR1 |= 1<<20; //Regular channel sequence length: 2

    ADC1->SQR3 |= 9; //1st conversion channel: IN9

    ADC1->SQR3 |= 8<<5; //2nd conversion channel: IN8

    ADC1->CR2 |= 1<<10; //Injection channel automatic conversion

    ADC1->CR2 |= 1<<23; //Turn on the temperature sensor

    ADC1->SMPR1 |= 5<<18; //SMP16[2:0] =5(55.5)

                                        // (55.5+12.5)/4MHz=17s

    ADC1->JSQR |= 0x10<<15; //JSRR[4:0]=0x10 (channel 16)

    ADC1->CR2 |= 1; //Turn on ADC

    ADC1->CR2 |= 1<<2; //Calibrate ADC

    while(ADC1->CR2 & 1<<2); //Wait for calibration to complete

}

ADC injection channel processing subroutine

//ADC1 processing subroutine

void Adc1_Proc(void)

{

    if(ADC1->SR & 2) //EOC=1 (conversion ends)

    {    

        adc1_dat[adc1_num] = ADC1->DR; //Read conversion value

        if(++adc1_num == 2)

            adc1_num = 0;

    }

    if(ADC1->SR & 4) //JEOC=1 (injection channel conversion ends)

    {

        ADC1->SR &= ~6; //Clear JEOC and EOC

        adc1_jdat = ADC1->JDR1; //Read conversion value

    }

}