Analog to digital converter (ADC) Pic18f2520

Analog to digital converter (ADC)

In this chapter we will discuss about microcontroller important peripheral Analog to digital converter. We will cover the entire basic configuration of ADC peripheral.

                              

You have seen on my past posts, we used pic18f2520 microcontroller. This controller has 10bit ADC modules inbuilt. A 10bit ADC means, it has a resolution of 10bits = 0 to 1023. Therefore, our best resolution is 1 part out of 1024. If we compare voltage vs resolution with ref(+) 5v and ref(-) 0v.

                                                                  

        Minimum sensing voltage =  ref(+)  - ref(-)

                                                                                 

                                                              1024

                                                   = 0.0048828125v

   

For example:-

                        ref(+)  = 5v

                        ref(-)   = 0v

         ADC input volt = 2.3v

              Digital value =  (1024 / (5-0)) *2.3 =  471

Basically microcontroller converts voltage to digital numerical value. That numerical value uses as the representation of the original input voltage while programming and you can use this value for various purposes in your program.

Second important point is ADC module can only be give conversion result when voltage is in defined reference range. Otherwise it will give a garbage value or can be damage. In many cases, the high and low voltage references are selected as the microcontroller supply voltage and ground, at other times an external reference or references are used.

ADC Block diagram

Pic18f2520 has ADC module registers:

PIC18f2520 ADC module has five registers. The ADCON0 register, shown in Register 19-1, controls the operation of the A/D module. The ADCON1 register, shown in Register 19-2, configures the functions of the port pins. The ADCON2 register, shown in Register 19-3, configures the A/D clock source, programmed acquisition time and justification.

• A/D Result High Register (ADRESH)

• A/D Result Low Register (ADRESL)

• A/D Control Register 0 (ADCON0)

• A/D Control Register 1 (ADCON1)

• A/D Control Register 2 (ADCON2)

ADCON0: A/D CONTROL REGISTER 0

        Bit6-7  :- Unimplemented: Read as ‘0’

          Bit5-2   :- Analog Channel Selection bits

·        We can select any one channel at a time ADC. These bits are important while you using multi-channel otherwise it will only configure one time.

Now we are using single ADC channel “AD0”.

ADCON0bits.CHS0 = 0;     //AD0 channel selection.

ADCON0bits.CHS1 = 0;

ADCON0bits.CHS2 = 0;

ADCON0bits.CHS3 = 0;

           Bit1    :- AD conversion status

·        This bits will tell that our ADC module is in working or free. Basically this bit use for monitor ADC module conversion. This bit should be clear (0) or can be use waiting for interrupt then check result registers.

ADCON0bits.Go

            Bit0   :- ADC on/off bit.

·        This bit uses for on-off the ADC module processing.

ADCON0bits.ADON = 1;   // AD conversion start

ADCON0bits.ADON = 0;   // AD conversion stop

ADCON1: A/D CONTROL REGISTER 1

        Bit7-6  :- Not in use

           Bit5    :- Voltage reference selection bit

·        We have discussed already that we can use reference voltage with in a valid range. There are two types of selection. First is the selection of according to microcontroller power supply and other is use external voltage reference.

ADCON1bits.VCFG1 = 1;  // Select external reference voltage range.

ADCON1bits.VCFG1 = 0;  // Select controller Vss reference voltage range.

            Bit4    :- Voltage reference selection bit

ADCON1bits.VCFG0 = 1;  // Select external reference voltage range.

ADCON1bits.VCFG0 = 0;  // Select controller Vdd reference voltage range.

            Bit3-0   :- Port pin selection as digital or analog input type.

·        The value in the bit 3 to 0 determines if pins are configured as analog or digital. Consult the data sheet of the device for a table that shows the allowable configuration options for port pins. We are using AD0 as analog input .

ADCON1bits.PCFG3 = 1;

ADCON1bits.PCFG2 = 1;

ADCON1bits.PCFG1 = 1;

ADCON1bits.PCFG0 = 0;

ADCON2: A/D CONTROL REGISTER 2

            Bit7   :- AD conversion valve format

·         This bit is important to get right result. Often we make mistake in define this bit correctly. The conversion value represent in two registers. The high and low result registers are ADRESH and ADRESL.

·        The result can be either left or right justified. When the result is left justified, the eight most significant bits will be placed in the ADRESH register with the least significant bits placed in the ADRESL register. When right justified, the eight least significant bits will be placed in the ADRESL register with the most significant bits placed in the ADRESH register. The selection of right or left justification will depend on the requirements of the user’s software.

ADCON2bits.ADMF = 1;   // Right justified.

ADCON2bits.ADMF = 0;   // Left justified.

            Bit6   :- Not in use

           

            Bit5-3   :- A/D Acquisition Time Select bits

·        Acquisition time is the ADC capacitor charging and discharging. If time is not sufficient then result will be garbage. The holding capacitor must be given sufficient time to settle to the analog input voltage level before the actual conversion is initiated.

·        See the equation 19-1,2,3 for more detail in controller datasheet.

·        Basically I always uses 12TAD.

ADCON2bits.ADQT = 1;

ADCON2bits.ADQT = 0;

ADCON2bits.ADQT = 1;

            Bit2-0   :- AD conversion clock selection

·        This time will be the analog to digital clock period multiplied by the number of bits of resolution in the analog to digital converter plus the two to three additional clock periods for the settling time.

·        There are two types of clock sources can be select. Main controller oscillator or AD RC oscillator. We will use fosc/64.

ADCONbits.ADCS2 = 1;

ADCONbits.ADCS1 = 1;

ADCONbits.ADCS0 = 0;

A/D Conversions

Now the time is to make software to read the RA0/AN0 pin analogical input voltage. We will connect a LED on PORTC0 and operate led if ADC conversion value reached on our set value. ADC uses many of the purposes in embedded field. Some of these are temperature display, liquid leveler and many more.

Circuit diagram:-

Software procedure flow diagram

Codes:-

#include <p18f2520.h>

#define LED PORTCbits.RC0        // Define LED PORT pin.

void ADC_result(void);

float ADC_data = 0;

float Set_Value = 2.35;

void main()

{

            TRISC = 0x00;          // PORTC as output port.

            TRISA = 0xFF;         // Configure PORTA pins as input.

           

            PORTC = 0x00;        // PORTC output 0 at starting time.

           

            ADCON0 = 0x01;     // ADC on, Channel-0 select.

            ADCON1 = 0x0E;    // Voltage reference VDD to Vss, AN-0 as analog input.

            ADCON2 = 0xAE;   // Right justified, 12 TAD, FOSC/64.

           

            while(1)

            {

                        ADC_result();          // Call to ADC checking function.

                        if(ADC_data > Set_Value)

                        {

                                    LED = 1;         // On LED if voltage cross limit value.

                        }                      

                        else

                        {

                                    LED = 0;

                        }

            }

}

void ADC_result()

{

            ADCON0bits.GO = 1;         // Set ADC conversion in progress.

            while( ADCON0bits.GO == 1 );   // Wait until conversion is not complete.

             // Convert AD value to voltage.

            ADC_data = (ADRESL + (ADRESH * 256)) / 204.8;   

} 

Download Project File :- https://drive.google.com/file/d/0B886Kbl42IVua1JvQkktMUFlVFU/view?usp=sharing