Arduino Nano Analog to Digital Converter

10 bit successive approximation ADC (Analog to Digital Converter):

• Converts a continuous analog waveform into a discrete digital representation
• Connected to an 8-channel Analog Multiplexer
• Convert analog input to 10 bit digital values (2^10 = 1024 steps) presented in the Data Register
• Min. value GND (0V), max. value reference voltage AREF
• The ADC has its own interrupt triggering when a conversion completes

Breadboard Setup

Arduino Analog Read

The Arduino function analogRead() reads an analog input signal from pins A[5:0]:

// read analog signal from pin A2
int value = analogRead(A2);
// convert into an 8-bit value
int value = map(value,0,1023,0,255);

The return value has 10-bit resolution, with value from 0 up to 1023.

AVR Analog Read

ADCSRA (ADC Control and Status Register):

Bit	Name	Description
7	ADEN	Enable (PORTA will behave as GPIO pins)
6	ADSC	Start Conversion (1 as long as conversion in progress)
5	ADATE	Auto Trigger Enable (triggered automatically at every rising edge of clock pulse)
4	ADIF	Interrupt Flag (1 after conversion finished)
3	ADIE	Interrupt Enable (1 to use interrupt-driven ADC)
2:0	ADPS[2:0]	Prescale select bits

// Enable ADC
ADCSRA |= (1<<ADEN);  
// Start single analog to digital conversions
ADCSRA |= (1<<ADSC);  

ADPS[2:0] (ADC Prescaler):

• The approximation circuitry requires an input clock frequency between 50kHz to 200kHz
• Frequency division factor produces the desired frequency from the CPU (order of MHz)
• Trade-off between frequency and accuracy

ADPS[2:0]	Division Factor
000	2
001	2
010	4
011	8
100	16
101	32
110	64
111	128

// Set ADC prescale sample rate @ 16MHz
ADCSRA |= (1<<ADPS2) | (1<<ADPS1) | (1<<ADPS0); // 16000000/128 = 125Khz
// Set ADC prescale sample rate @ 1MHz
ADCSRA |= (0<<ADPS2) | (1<<ADPS1) | (1<<ADPS0); // 1000000/8 = 125Khz

SFIOR (Special Function I/O Register):

• ADATE=1 (in ADCSRA in register)
• SFIOR[7:5] determine the trigger source for ADC conversion

Multiplexer Selection

ADMUX (ADC Multiplexer Selection) register:

Bit	Name	Description
7:6	REFS[1:0]	Reference voltage selection
5	ADLAR	ADC Left Adjust Result (1’ to left adjust)
4	RES	Reserved (always 0)
3:0	MUX[3:0]	Analog input channel and gain selection bits for PA[0:7]

REFS[1:0] voltage reference selection:

REFS1	REFS0	Description
0	0	Connect AREF to external reference voltage source (internal Vref off)
0	1	AVCC (+5V) with external capacitor at AREF pin
1	0	Reserved
1	1	Internal Vref (+2.66V) reference with external capacitor on AREF pin

MUX[3:0] analog input channel selection:

• Single-ended voltage inputs PORTA (pins PA[0:7])

MUX[3:0]	Input
0000	ADC0
0001	ADC1
0010	ADC2
0011	ADC3
0100	ADC4
0101	ADC5
0110	ADC6

ADMUX |= (1<<REFS0); // Set ADC reference to AVCC
ADMUX |= (1<<ADLAR); // Left adjust ADC result to allow easy 8 bit reading

Data Registers

After ADC conversion is complete the data registers present the result:

• Default right adjusted 10-bit conversion result:
  • Results from 0 up to 1023 (2^10 = 1024 steps)
  • Read ADC (combines ADCL (ADC Low byte) and ADCH (ADC High byte))
• Use a left adjusted 8-bit conversion result:
  • Results from 0 up to 254 (2^8 = 255 steps)
  • Set ADMUX.ADLAR to 1, sufficient to read ADCH
• ADC = VIN*1024/VREF (VIN voltage on input pin, VREF voltage reference)

Read a 10-bit conversion result:

// start single analog to digital conversions
ADCSRA |= (1<<ADSC);
// wait for conversion to complete
while(!(ADCSRA & (1<<ADIF)));
// read the conversion result
int value = ADC; // (ADCH<<8)|ADCL
// clear ADIF by writing one to it
ADCSRA |= (1<<ADIF);

Read an 8-bit conversion result:

// left adjust result 
ADMUX |= (1<<ADLAR);
// start single analog to digital conversions
ADCSRA |= (1<<ADSC);
// wait for conversion to complete
while(!(ADCSRA & (1<<ADIF)));
// read only the high-byte
int value = ADCH;
// clear ADIF by writing one to it
ADCSRA |= (1<<ADIF);