Chapter 9 – Digital Meets Analog – ADC and DAC

Projects

We will write code to allow us to use HyperTerminal to request a reading from the light, temperature and voltage sensors of the Butterfly. You’ve already seen the debugging tale above so you know how much fun I had writing this stuff, so enjoy it or else.

Initializing the ADC

The Butterfly has the ATmega169 pin 62, (AREF) connected to a bypass capacitor to help lessen noise on the ADC, so we set the ADMUX bits 6 and 7 to 0 to select the 'use external reference' option. We use the ‘input’ variable to set the multiplexer. to connect the ADC to pin 61 (ADC0) using the ADMUX register (data book p 207).

Next we set the ADC Control and Status Register A. The ADEN bit enables the ADC. The ADPSx bits select the prescaler.

// set ADC prescaler to , 1MHz / 8 = 125kHz ADCSRA = (1<<ADEN) | (1<<ADPS1) | (1<<ADPS0);

Finally we take a dummy reading, which basically allows the ADC to hack up any hairballs before we take any real readings

input = ADC_read();

void ADC_init(char input)

{

// set ADC prescaler to , 1MHz / 8 = 125kHz ADCSRA = (1<<ADEN) | (1<<ADPS1) | (1<<ADPS0);

input = ADC_read(); // clear hairballs

}

216

Chapter 9 – Digital Meets Analog – ADC and DAC

Reading the ADC

We save power by turning off the voltage on the light and temperature sensors when they are not used, so now we turn them on, in case they are being used.

sbi(PORTF, PF3); sbi(DDRF, DDF3);

Next we enable the ADC.

Then we do another hairball clearing dummy read.

And we wait till the conversion is complete.

while(!(ADCSRA & 0x10));//wait for conversion done, ADIF flag active

Now we repeat this 8 times for better accuracy.

// do the ADC conversion 8 times for better accuracy for(i=0;i<8;i++)

{

ADCSRA |= (1<<ADSC); // do single conversion

// wait for conversion done, ADIF flag active while(!(ADCSRA & 0x10));

ADC_temp = ADCL; // read out ADCL register ADC_temp += (ADCH << 8); // read out ADCH register

// accumulate result (8 samples) for later averaging ADCr += ADC_temp;

}

We divide by 8, which conveniently is done by left shifting 3 bits. Weren’t we lucky that we chose to do 8 samples and save processing time by avoiding a division?

ADCr = ADCr >> 3; // average the 8 samples

217

Chapter 9 – Digital Meets Analog – ADC and DAC

We turn the sensors off to save power.

cbi(PORTF,PF3); // mt cbi(PORTF, PORTF3); // disable the VCP cbi(DDRF,DDF3); // mt cbi(DDRF, PORTF3);

And we disable the ADC and return the calculated value.

return ADCr;

Giving us the ADC_read function:

int ADC_read(void)

{

char i;

int ADC_temp;

//mt int ADC = 0 ; int ADCr = 0;

//To save power, the voltage over the LDR and the NTC is

//turned off when not used. This is done by controlling the

//voltage from an I/O-pin (PORTF3)

sbi(PORTF, PF3); // Enable the VCP (VC-peripheral) sbi(DDRF, DDF3); // sbi(DDRF, PORTF3);

//wait for conversion done, ADIF flag active while(!(ADCSRA & 0x10));

//do the ADC conversion 8 times for better accuracy for(i=0;i<8;i++)

{

218

Chapter 9 – Digital Meets Analog – ADC and DAC

// accumulate result (8 samples) for later averaging ADCr += ADC_temp;

}

ADCr = ADCr >> 3; // average the 8 samples

cbi(PORTF,PF3); // disable the VCP cbi(DDRF,DDF3); // mt cbi(DDRF, PORTF3);

return ADCr;

}

Light Meter

The Butterfly has a Light Dependent Resistor, LDR, connected to ADC channel 2. The resistance of the LDR decreases as the light increases, so the voltage measured will decrease as light decreases.

We write the getLight function:

void getLight()

{

char light[]= {'0','0','0','\0'}; int ADCresult = 0;

// Initialize the ADC to the light sensor channel ADC_init(2);

ADCresult = ADC_read();

itoa(ADCresult, light, 10);

// Send the temperature to the PC sendString("The light reading is "); sendString(light);

sendString(" somethings.\r");

}

This is straightforward and returns a value for the light. The light units ‘somethings’ is a precise scientific measure that means: ‘I don’t have a clue as to

219