Analog to Digital Converter with PIC Microcontroller (PIC16F877A)

Why need ADC? !!!!

ADC pins of PIC16F877A

Features of PIC16F877A ADC:

• 10-bit 8 channels analogue input ADC
• ±2 LSB absolute accuracy
• Up to 15 KSPS at Maximum Resolution
• 0-Vcc ADC input votage range 
• Reference Voltage can be configured as Avcc, AREF or 2.56 Volts
• Sleep Mode Noise Canceller. 

What is Resolution?

• The smallest change that can be detected in the measuring voltage.
• 10 Bit resolution provide 210 =1024 steps.
• Vcc = 5Volt
• Step Size(mV) = 5/1024 = 4.88

Conversion Time:

• Conversion time is defined as the time an ADC takes to convert the analogue input into the digital (binary) output.
• The conversion time depends on -MCU clock source.

ADC Timings:

• Startup time: should be considered when the ADC has been enabled either for the first time or after a wake up from some of the sleep modes.
• Sample and hold time: This time must be considered carefully especially when multiple channels are used during conversion.

The A/D converter module has the following features:

      The converter generates a 10-bit binary result using the method of successive approximation and stores the conversion results into the ADC registers (ADRESL and ADRESH). There are 14 separate analog inputs; The A/D converter converts an analog input signal into a 10- bit  binary number. The minimum resolution or quality of conversion may be adjusted to various needs by selecting voltage referances Vref- and Vref+.

ADC programming in PIC16F877A:

          The A/D module has four registers. These registers are:

• A/D Result High Register (ADRESH)
• A/D Result Low Register (ADRESL)
• A/D Control Register 0 (ADCON0)
• A/D Control Register 1 (ADCON1)

ADRESH and ADRESL Registers:

The result obtained after converting an analog value into digital is a10-bit number that is to be stored in the ADRESH and ADRESL registers. There are two ways of handling it – left and right justification which simplifies its use to a great extent. The format of conversion result depends on the ADFM bit of the ADCON1 register. In the event that the A/D converter is not used, these registers may be used as general-purpose registers.

ADCON0 Register:

ADCS1, ADCS0 – A/D Conversion Clock Select bits select clock frequency used for internal synchronization of A/D converter. It also affects duration of conversion.

CHS3-CHS0 – Analog Channel Select bits select a pin or an analog channel for A/D conversion, i.e. voltage measurement:

GO/DONE – A/D Conversion Status bit determines current status of conversion:

1 – A/D conversion is in progress.

0 – A/D conversion is complete. This bit is automatically cleared by hardware when the A/D conversion is complete.

ADON – A/D On bit enables A/D converter.

1 – A/D converter is enabled.

0 – A/D converter is disabled.

ADCON1 Register:

To do an A/D Conversion, follow these steps:

Step 1 --Configure the A/D module:

• Configure analog pins/voltage reference and digital I/O(ADCON1)
• Select A/D input channel (ADCON0)
• Select A/D Conversion clock (ADCON0)
• Turn on A/D module (ADCON0)
• Select one of input channels CH0-CH13 of the ADCON0 register
• Select data format using the ADFM bit of the ADCON1 register.
•  Enable A/D converter by setting the ADON bit of the ADCON0 register.

Step 2 – ADC interrupt configuration (optionally):

                Clear the ADIF bit.

                Set the ADIE, PEIE and GIE bits.

Step 3 – Wait for the required acquisition time to pass (approximately 20uS).

Step 4 – Start conversion by setting the GO/DONE bit of the ADCON0 register.

Step 5 – Wait for ADC conversion to complete.

                It is necessary to check in the program loop whether the GO/DONE pin is cleared or wait                      for an A/D interrupt (must be previously enabled).

Step 6 – Read ADC results:

                Read the ADRESH and ADRESL registers.

Circuit Diagram:

REMEMBER !!!!

• The AVCC and VREF should be constant. If they vary, the accuracy of ADC conversion will be affected.
• Input must not exceed VCC
• Signal components higher than the Nyquist frequency (fADC/2) should not be present for either kind of channels, to avoid distortion from unpredictable signal convolution.

Basic code:

Define CONF_WORD = 0x3f72

Define CLOCK_FREQUENCY = 12

AllDigital

ADCON1 = 0x0e

Define LCD_BITS = 8

Define LCD_DREG = PORTD

Define LCD_DBIT = 0

Define LCD_RSREG = PORTE

Define LCD_RSBIT = 0

Define LCD_RWREG = PORTE

Define LCD_RWBIT = 1

Define LCD_EREG = PORTE

Define LCD_EBIT = 2

Define LCD_READ_BUSY_FLAG = 1

Lcdinit

Dim an0 As Word

loop:

  Adcin 0, an0

  Lcdcmdout LcdClear

  Lcdout "Analog input AN0"

  Lcdcmdout LcdLine2Home

  Lcdout "Value: ", #an0

  WaitMs 250

Goto loop

C Code

// LCD module connections

 sbit LCD_RS at RD0_bit;

 sbit LCD_EN at RD1_bit;

 sbit LCD_D4 at RD4_bit;

 sbit LCD_D5 at RD5_bit;

 sbit LCD_D6 at RD6_bit;

 sbit LCD_D7 at RD7_bit;

sbit LCD_RS_Direction at TRISD0_bit;

sbit LCD_EN_Direction at TRISD1_bit;

sbit LCD_D4_Direction at TRISD4_bit;

sbit LCD_D5_Direction at TRISD5_bit;

sbit LCD_D6_Direction at TRISD6_bit;

sbit LCD_D7_Direction at TRISD7_bit;

 // End LCD module connections

int adc;

char adc_con[7];

void main()

{

ADCON0 = 0x41; //ADC Module Turned ON and Clock is selected

ADCON1 = 0xC0; //All pins as Analog Input

TRISA.RA0=1;      // Configure RA0 pin as input

ADC_Init();         // Initialize ADC

Lcd_Init();          // Initialize LCD

Lcd_Cmd(_LCD_CLEAR);    // Clear display

lcd_cmd(_LCD_CURSOR_OFF);

lcd_out(1,1,"ADC Reading");

delay_ms(1000);             // 1 Second delay

Lcd_Cmd(_LCD_CLEAR);  // Clear display

while(1)

{

adc = ADC_Read(0);          // Get 10-bit results of AD conversion

IntToStr(adc, adc_con);               // Convert temperature to string

lcd_out(1,1,"ADC Valu=");

Lcd_Out(1, 10, adc_con);                   // Display Temperature

 Delay_ms(500);                     // 1 Second delay

}

}

Circuit Diagram: