Application of AVR MCU ADC

Application of AVR MCU ADC

1. Basic knowledge of digital-to-analog conversion

The digital-to-analog converter (ADC) is basically a matter of ratios, that is, the digital value produced by the ADC is related to the ratio of the input analog value to the converter range. The conversion relationship is as follows:

Vin/Vfullscale=X/(2N-1) where X is the digital output, N is the number of bits of the digital output (the number of bits of the ADC), Vin is the value of the analog input, and Vfullscale is the maximum value of the analog input.

Calculation of conversion accuracy of digital-to-analog conversion:

Vresolution=Vfullscale/((2N-1)

2. Introduction to ADC of AVR microcontroller

The ADC of the AVR microcontroller has two ADC registers: the ADC control and status register ADCSR controls the operation of the ADC; the ADC multiplexer selector ADMUX controls the 8 measured analog inputs.

In order for the ADC to operate with maximum accuracy, a clock frequency between 50HZ and 200K HZ is required, and a suitable proportional division factor is selected to obtain a conversion frequency of 50HZ to 200KHZ. Since the ADC is relatively slow, if the processor is in a waiting state while the ADC is converting data, it will waste time, so the ADC usually uses interrupt mode. ADC initialization:

(1) Set the lowest three bits of ADCSR to determine the frequency division factor

(2) Set ADIE to 1 to enable interrupt mode

(3) Set ADEN to high level to enable ADC

(4) Set ADSC to start conversion immediately

3. Application

System function: When the input voltage of analog channel 3 exceeds 3V, the system lights up the red LED; when the input voltage is less than 2V, the yellow LED is lit; when the input voltage is between 2V and 3V, the green LED is lit.

Hardware connection: The three LEDs are connected to PORTD.0, PORTD.1, and PORTD.2 of the PORD port of AT908535. The ADC peripheral of the AVR microcontroller will vary depending on the specific microcontroller used. All ADCs have noise suppression requirements for the pin voltage in the VCC connection. Some have built-in noise eliminators, while others have the ability to control Vref internally. When using ADC, you need to check the manual of the relevant microcontroller.

Program code:

#i nclude<90s9535.h>

#define LEDS PORTD

#define red 0xfe

#define yellow 0xfd

#define green 0xfc

#define uchar unsigned char

interrupt[ADC_INT] void adc_isr(void)

{

uchar adc_data; //variable for ADC result

adc_data=ADCW; //read all 10bits into variable

if(adc_data>(3*1023)/5)

LEDS=red;

else(adc_data<(2*1023)/5)

LEDS=yellow;

else

LEDS=green;

ADCSR=ADCSR|0x40; //start the next conversion

}

void main()

{

DDRD=0x07; //least signifcant 3 bits for output

ADMUX=0x03; //select to read only channel 3

ADCSR=0xCE; //ADC interrupt, 64 division

#asm("sei")

while(1); //Wait for ADC interrupt to occur

}