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voltage=analog2v(convertanalog(0));
write(PSTR("Akumulátor: "));
writestr(voltage2float(voltage));

void putchar1(char c) {
	while (!(UCSRA & DATA_REGISTER_EMPTY));
	UDR=c;
}

void write(char *sss){
	char k;
	while ((k=pgm_read_byte(sss++))) {
		putchar1(k);
	}
}

void writestr(char *sss){
	char k;
	while ((k=(*sss++))) {
		putchar1(k);
	}
}

analog.c

/* vim: set sw=8 ts=8 si et: */
/*
* ADC functions for atmega8.
* Author: Guido Socher, Copyright: GPL 
* http://tuxgraphics.org/electronics/
*/
#include <avr/io.h>

//----------------EDIT HERE----------------------------------------
// VDIV = (Rx+Ry)/Rx, change this according to the division factor of Rx and Ry
//
// This is if you do not use any resistor for Rx and Ry=4K7 (measure up to 2.5V):
//#define VDIV 1.0
//
// Ry=4K7 Rx=4K7 -> divide by 2 (measure up to 5V)
#define VDIV 7.745

// convert adc reading to voltage 
unsigned int analog2v(unsigned int aval)
{
        float r;
        // 100* 2.56*VDIV/1024:
        r=(aval * VDIV )/4.0;
        return((unsigned int)(r+0.5));
}

//-------------END EDIT HERE----------------------------------------

// return analog value of a given channel. Works without interrupt 
unsigned int convertanalog(unsigned char channel) 
{
        unsigned char adlow,adhigh;
        /* enable analog to digital conversion in single run mode
        *  without noise canceler function. See datasheet of atmega88 page 250
        * We set ADPS2=1 ADPS0=1 ADPS1=1 to have a clock division factor of 128.
        * This is needed to stay in the recommended range of 50-200kHz 
        * Clock freq= 14MHz or 18 MHz
        * ADEN: Analog Digital Converter Enable
        * ADIE: ADC Interrupt Enable  (0=no interrupt)
        * ADIF: ADC Interrupt Flag
        * ADCSR: ADC Control and Status Register
        * ADPS2..ADPS0: ADC Prescaler Select Bits
        * REFS: Reference Selection Bits (page 203)
        */

        // int-ref with external capacitor at AREF pin: 
        // atmega8: 2.56V int ref=REFS1=1,REFS0=1
        // atmega88: 1.1V int ref=REFS1=1,REFS0=1
        // write only the lower 3 bit for channel selection
        //
#ifdef USEAVCCREF
        // AVcc ref
        ADMUX=(1<<REFS0)|(channel & 0x0f);
#else
        // Use the intenal  ref:
        ADMUX=(1<<REFS1)|(1<<REFS0)|(channel & 0x0f);
#endif
        // 
        ADCSRA=(1<<ADEN)|(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);
        // switch off digital input line:
        //DIDR0=(1<<channel)& 0x1f;

        //  start conversion 
        ADCSRA|= (1<<ADSC);
        while(bit_is_set(ADCSRA,ADSC)); // wait for result 

        adlow=ADCL; // read low first !! 
        adhigh=ADCH; 
        return((unsigned int)((adhigh<<8)|(adlow & 0xFF)));
}


analog.h


/* vim: set sw=8 ts=8 si et : */
/*
* Title	:   C include file for analog conversion
* Copyright: GPL V2
* Autor: Guido Socher
* http://tuxgraphics.org/electronics/
*/
#ifndef ANALOG_H
#define ANALOG_H

// return analog value of a given channel.
extern unsigned int convertanalog(unsigned char channel);
extern unsigned int analog2v(unsigned int aval);

#endif /* ANALOG_H */