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

Summary

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.

Description

 

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
 
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)
 
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
A
A
A
A
A
A
A
A
A
A
A
A
A
0001
A
A
A
A
A
A
A
A
A
A
A
A
A
0010
A
A
A
A
A
A
A
A
A
A
A
A
A
0011
D
A
A
A
A
A
A
A
A
A
A
A
A
0100
D
D
A
A
A
A
A
A
A
A
A
A
A
0101
D
D
D
A
A
A
A
A
A
A
A
A
A
0110
D
D
D
D
A
A
A
A
A
A
A
A
A
0111
D
D
D
D
D
A
A
A
A
A
A
A
A
1000
D
D
D
D
D
D
A
A
A
A
A
A
A
1001
D
D
D
D
D
D
D
A
A
A
A
A
A
1010
D
D
D
D
D
D
D
D
A
A
A
A
A
1011
D
D
D
D
D
D
D
D
D
A
A
A
A
1100
D
D
D
D
D
D
D
D
D
D
A
A
A
1101
D
D
D
D
D
D
D
D
D
D
D
A
A
1110
D
D
D
D
D
D
D
D
D
D
D
D
A
1111
D
D
D
D
D
D
D
D
D
D
D
D
D

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)
 
 
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
 
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). (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.

 

Circuit Diagram

Video

Code

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