fft_led/avr_adc.c

//setup adc for an attiny85
//
#include<avr/io.h>
#include"avr_adc.h"
#define _BS(bit) (1<<bit)
#define _BC(bit) ~(1<<bit)
#define STARTADC ADCSRA |= _BS(ADSC)


		//1.setup_prescaler
		  /* this function initialises the ADC

        ADC Prescaler Notes:
	--------------------

	   ADC Prescaler needs to be set so that the ADC input frequency is between 50 - 200kHz.

           For more information, see table 17.5 "ADC Prescaler Selections" in
           chapter 17.13.2 "ADCSRA – ADC Control and Status Register A"
          (pages 140 and 141 on the complete ATtiny25/45/85 datasheet, Rev. 2586M–AVR–07/10)

           Valid prescaler values for various clock speeds

	     Clock   Available prescaler values
           ---------------------------------------
             1 MHz   8 (125kHz), 16 (62.5kHz)
             4 MHz   32 (125kHz), 64 (62.5kHz)
             8 MHz   64 (125kHz), 128 (62.5kHz)
            16 MHz   128 (125kHz)

           Below example set prescaler to 128 for mcu running at 8MHz
           (check the datasheet for the proper bit values to set the prescaler)
  */
void setupADC(void)
{
		ADMUX = //_BC(REFS0) |	//Set_Vref 
				//_BC(REFS1) |	//Set_Vref
				//_BC(MUX3)  |	//
				//_BC(MUX2)  |	//
			//	_BS(MUX1)  |	//
				_BS(MUX0)  |	//PORT Select PB3
				_BS(ADLAR); 		//left shift conversion results(used in 8bit mode)
	//	ADCSRB = _BS(
		ADCSRA = _BS(ADEN) |	//Enable ADC
				//_BC(ADATE) |	//Disable AutoTrigger aka Freerunning mode
			//	_BS(ADPS1) |	//Select Prescaler
				_BS(ADPS0) |	//prescaler 128
				//_BS(ADPS1) |
				_BS(ADPS2);
		ADCSRA &= ~(1<<ADATE);
}