How to work with inbuilt ADC Module of PIC Microcontroller (PIC18F4550)

·         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.

 

 

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)

 

 

 

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

 

 

2. ADCON1 (A/D CONTROL REGISTER 1)

 

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):

 

 

 

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

 

 

 

3. ADCON2 (A/D CONTROL REGISTER 2)

 

 

 

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:

 

 

 

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)

Project Source Code

###

// 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 5×7 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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