Atmega328p-based DC-AC converter to ring a T65 phone, simpler hardware, but horrible waveshape

main.c

#include <avr/io.h>
#include <avr/interrupt.h>
#define F_CPU 8e6
#include <util/delay.h>

/*
FUSES = {
	.low = 0xE2, //INTRC 8 MHZ, no CKDIV8
	.high = 0xD9, // geen bootloader
	.extended = 0xFF, // geen BOD
};

LOCKBITS = 0xFF; // {LB=PROG_VER_DISABLED, BLB0=LPM_SPM_DISABLE, BLB1=LPM_SPM_DISABLE}
*/

int16_t setpoint = 128; //With 1.1V internal ADC reference and a 8 bit result, 128 means 0.55 V approximately for the output divider
const uint8_t sinus[64]={127,139,152,164,176,187,198,208,217,226,233,239,245,249,252,254,255,254,252,249,245,239,233,226,217,208,198,
187,176,164,152,139,127,115,102,90,78,67,56,46,37,28,21,15,9,5,2,0,0,0,2,5,9,15,21,28,37,46,56,67,78,90,102,115};

void setup()
{
    /*GPIO*/
    DDRC = 0xBE;                  // PC0 input, PC6=reset input, rest ouput
    DDRB = 0x3F;                  // all output, except crystal input PB6,7
    DDRD = 0xF7;                  // all output, except PD3 (switch?)
 
    /* Timer 1, for PWM output and ADC samplerate (PWM /8) 
     * Has to be set to inverted output, because otherwise OCRA1 == 0 results in a small spike
     * on inverted, OCR1A == TOP == ICR1 means a constantly low signal and OCR1A = 0 a nearly constantly high signal
     */
    ICR1 = 80;                    // use 80 as top, so 8M/80=100 kHz PWM
    TCCR1A = 0b11000010;          // use OC1A output (inverted), fast PWM, ICR1 as top
    TCCR1B = 0b00011001;          // WGM mode fast PWM ICR1 as top, clock timer from clkIO unprescaled, start
    TIMSK1 = (1<<TOIE0);          // enable interrupt on overflow (at ICR1 value)
  
    /*ADC*/
    ADMUX = 0b11100000;           // set reference voltage to 1V1 internal, use ADC0 as input, left adjust result (so only higher 8 bits have to be read)
    ADCSRA = (1<<ADEN)|(1<<ADIE)|(1<<ADPS2)|(1<<ADPS0); // ADC enable, interrupt enable, clock source clkio/32 (8M/32 < 200kHz) 
    DIDR0 = 0x01;                 // disable digital input buffer on ADC0 input
       
    sei();                        // enable interrupts
}

int main(void) {
    uint8_t i=0;
    /*  */
    setup();
    while (1) {
        i++;
        if(i>63) i=0;
        setpoint = sinus[i];
        _delay_us(257);           // TODO: tune to get 20 Hz
    }
}

ISR(TIMER1_OVF_vect){
    static uint8_t i=0;
    i++;
    if(i>=7){ 
     ADCSRA |= (1<<ADSC); // start a new adc conversion every 8th OVF. OVF at 100 kHz, ADC max is 15ksps at max resolution
     i=0;
    }
    PIND = 0x01; // toggle PD0 for debug
}

ISR(ADC_vect){
    /* 
     * read adc result, and set new PWM value
     * because PWM is inverted to prevent the 1 cycle high output
     * 80 means output LOW, 0 means 100% dutycycle high,
     * 23 means 70% dutycycle high.
     * 
     * If PWM where not inverted an OCR1A 0 would mean 0 output instead of
     *  a thin needle of 1 clk cycle, then this would be a bit simpler:
     * if error > 0 {if error<56 OCR=error }else ocr=56 else ocr = 0;
     */
    uint8_t adcresult = ADCH; 
    int16_t error = (setpoint - adcresult)*8; // P factor
    if(error>0){
        if(error < 57) OCR1A = 80-error; else OCR1A = 40;  // limit to 50% dutycycle (40) max
    }else OCR1A = 80;
}