How to work with inbuilt ADC Module of PIC Microcontroller (PIC18F4550)- (Part 11/25)

A microcontroller, a digital device, can read, execute and transmit only digital signals. On the contrary, the outputs of the most of the transducers are analog in nature. Thus it is hard to interface these transducers directly with controllers. Analog-to-digital convertor (ADC) ICs are one way to make the analog input compatible with the microcontroller.

Using an external ADC adds complexity to the circuit. To avoid this complexity, PIC Microcontrollers have in-built ADC module which reduces the cost and connections of the circuit. This article explains the in-built ADC of PIC18F4550 controller.

As mentioned in the summary, a PIC microcontroller has inbuilt ADC for A/D conversion. The ADC module of PIC18F4550 controller has following specifications:

· 10-bit resolution output which means that an analog input gets converted into a corresponding 10-bit digital output.

· 13 channels which means that a total of 13 analog signals can be converted simultaneously into digital.

· Vref+ (RA3) and Vref- (RA2) pins for external reference voltage.

· 8 selectable clock options.

· ADC can be in auto-triggering mode for continuous A/D conversion.

ADC Registers:

To work with the inbuilt ADC of this PIC microcontroller, the certain registers are required to be configured. Each of these ADC registers has been explained below.

1. ADCON0 (A/D CONTROL REGISTER 0)

Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
—	—	CHS3	CHS2	CHS1	CHS0	GO/DONE	ADON

Fig. 2: Bit Configuration of ADCON0 Control register enable/disable the ADC peripheral of PIC

ADON: This bit is used to enable/disable the ADC peripheral of the PIC.

1 = A/D converter module is enabled

0 = A/D converter module is disabled

GO/DONE: This is A/D conversion status bit. For ADON=1,

1 = A/D conversion in progress

0 = A/D Idle

CHS3: CHS0: These bits are used to select a particular analog channel from 13 available channels (0-12) which are multiplexed with digital I/O pins. The following table shows the bit configuration to select these analog channels:

CHS3:CHS0	Analog Channel	Pin
0000	Channel 0	RA0/AN0
0001	Channel 1	RA1/AN1
0010	Channel 2	RA2/AN2
0011	Channel 3	RA3/AN3
0100	Channel 4	RA5/AN4
0101	Channel 5	RE0/AN5
0110	Channel 6	RE1/AN6
0111	Channel 7	RE2/AN7
1000	Channel 8	RB2/AN8
1001	Channel 9	RB3/AN9
1010	Channel 10	RB1/AN10
1011	Channel 11	RB4/AN11
1100	Channel 12	RB0/AN12

Fig. 3: Bit configuration to select analog channels in PIC

2. ADCON1 (A/D CONTROL REGISTER 1)

Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
—	—	VCFG1	VCFG0	PCFG3	PCFG2	PCFG1	PCFG0

PCFG0:PCFG3: As mentioned earlier, there are 13 analog channels in PIC18F4550 which are multiplexed with digital I/O pins. This means that such a (multiplexed) pin can act as either a digital I/O pin or an analog input pin. Either of these configurations is selected by these bits. The following table shows the bit configuration to make a pin D (Digital I/O) or A (Analog input):

PCFG3:

PCFG0

AN12

AN11

AN10

AN9

AN8

AN7

AN6

AN5

AN4

AN3

AN2

AN1

AN0

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

1010

1011

1100

1101

1110

1111

Fig. 4: Bit configuration to make pin D (Digital I/O) or A (Analog input) in PIC

VCGF1-VCGF0: These are voltage configuration bits to select the reference voltage (Vref) for ADC module.

Bit	= 1	= 0
VCGF0	Vref+ (RA3)	Vcc
VCGF1	Vref- (RA2)	Vss (GND)

Fig. 5: Voltage configuration bits for reference voltage (Vref) in ADC module

3. ADCON2 (A/D CONTROL REGISTER 2)

Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
ADFM	—	ACQT2	ACQT1	ACQT0	ADCS2	ADCS1	ADCS0

ADCS2:ADCS0: These bits are used to select the clock option for ADC peripheral. The following table shows the bit configuration to select from different clock options:

ADCS2:ADCS0	Clock Option
000	FRC (clock derived from A/D RC oscillator)
001	FOSC/64
010	FOSC/16
011	FOSC/4
100	FRC (clock derived from A/D RC oscillator)
101	FOSC/32
110	FOSC/8
111	FOSC/2

Fig. 6:Bit configuration to select clock for ADC peripheral

ACQT2:ACQT0: These bits are used to set the acquisition time of the ADC. The acquisition time is the time required to charge and discharge the holding capacitor of the ADC.

ADFM: This bit is used to select the format of digital output.

1 = Right justified (LSB to MSB)

0 = Left justified (MSB to LSB)

4. ADRESL & ADRESH:

Since the ADC module of PIC provides 10-bit digital output after A/D conversion, this output is stored in two 8-bit registers, namely, ADRESL & ADRESH. The lower byte is stored in ADRESL (A/D Result High register) while the higher byte is stored in ADRESH (A/D Result Low register).

Working with ADC

Objective: To select an analog channel of PIC18F4550’s in-built ADC and provide an analog input (0 to 5 volt) to it using a variable resistor or preset (20 k)

Fig. 6: Symbol of Ohm

(See circuit diagram) The main objective is to read the analog signal and display the corresponding digital value on LCD. (Also see interfacing LCD with PIC)

Programming Steps:

1. Set number of analog inputs by setting PCFG3:PCFG0 (ADCON1).

2. Select the analog channel by setting CHS3:CHS0 bits (ADCON0).

3. Select the clock option, acquisition time output format by configuring the ADCON2 register.

4. Enable the ADC by making ADON bit (ADCON0) high.

5. Start the conversion by setting GO/DONE bit (ADCON0).

6. Wait until GO/DONE bits becomes low. This indicates that the A/D conversion is over.

7. Store A/D conversion result from ADRESH:ADRESL into a variable.

8. Covert the resultant value to its corresponding ASCII value and display on LCD.

Project Source Code

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// Program to depict working with inbuilt ADC of PIC18F4550 Microcontroller
// This code uses Channel0 (zero) of PIC's ADC Module

// Configuration bits
/* _CPUDIV_OSC1_PLL2_1L,  // Divide clock by 2
   _FOSC_HS_1H,           // Select High Speed (HS) oscillator
   _WDT_OFF_2H,           // Watchdog Timer off
   MCLRE_ON_3H            // Master Clear on
*/

#define rs LATA.F0
#define rw LATA.F1
#define en LATA.F2
#define lcdport LATB

void lcd_ini();
void lcdcmd(unsigned char);
void lcddata(unsigned char);
void adc_con(unsigned int);
void adc_init();

unsigned char data[20]="ADC OUTPUT=";
unsigned int digital_out[10],avg_output=0,temp;
unsigned int i=0;


void main()
{
    TRISA=0x01;        // Configure RA0 as input pin
    LATA=0;
    TRISB=0;        // Configure Port B as output port
    LATB=0;
    TRISD=0;
    LATD=0;
    lcd_ini();        // LCD initialization
    while(data[i]!='')
    {
        lcddata(data[i]);      // Call lcddata function to send character one by from 'data' array
        i++;
    }

    adc_init();        //ADC Initialization

    while(1)
    {
        temp=0;
        for(i=0;i<10;i++)
        {
            ADCON0|=(1<<GO);                              // Start A/D conversion
            while(!(ADCON0 & (1<<GO)));                   // Wait until conversion gets over
            digital_out[i]=((ADRESL)|(ADRESH<<8));        // Store 10-bit output into a 16-bit variable
            Delay_ms(20);
            temp=temp+digital_out[i];
        }
        avg_output=temp/10;                           // Take average of ten digital values for stablity
        adc_con(avg_output);                          // Function to convert the decimal vaule to its corresponding ASCII

    }
}


void adc_init()
{
    ADCON1=0x0E;                            // Make RA0/AN0 pin as analog pin (Other pins remain to be digital I/O)
    ADCON0=0x00;                            // Select Channel0 & ADC off
    ADCON2=0x8A;                            // Left justified, 2TAD acquiciation time, Fosc/32 clock option
    ADCON0.ADON=1;                          // Enable ADC
}


void lcd_ini()
{
    lcdcmd(0x38);        // Configure the LCD in 8-bit mode, 2 line and 5x7 font
    lcdcmd(0x0C);        // Display On and Cursor Off
    lcdcmd(0x01);        // Clear display screen
    lcdcmd(0x06);        // Increment cursor
    lcdcmd(0x80);        // Set cursor position to 1st line, 1st column
}


void adc_con(unsigned int adc_out)
{
    unsigned int adc_out1;
    int i=0;
    char position=0xC3;

    for(i=0;i<=3;i++)
    {
        adc_out1=adc_out%10;                     // To exract the unit position digit
        adc_out=adc_out/10;
        lcdcmd(position);
        lcddata(48+adc_out1);                    // Convert into its corresponding ASCII
        position--;

    }
}


void lcdcmd(unsigned char cmdout)
{
    lcdport=cmdout;        //Send command to lcdport=PORTB
    rs=0;                        
    rw=0;
    en=1;
    Delay_ms(10);
    en=0;
}


void lcddata(unsigned char dataout)
{
    lcdport=dataout;    //Send data to lcdport=PORTB
    rs=1;
    rw=0;
    en=1;
    Delay_ms(10);
    en=0;
}
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