ATtiny13A instruction use summary:
.lds  :   0 .sts  :   0

Table 4-1. Versions of AVR® 8-bit CPU
Name           Description
AVR        Original instruction set from 1995
AVRe       AVR instruction set extended with the Move Word (MOVW) instruction, and the Load Program
                  Memory (LPM) instruction has been enhanced. Same timing as AVR.
AVRe+     AVRe instruction set extended with the Multiply (xMULxx) instruction. Same timing as AVR and
                  AVRe.
AVRxm     AVRe+ instruction set extended with the Read Modify Write (RMW) and Data Encryption
                 Standard (DES) instructions. SPM extended to include SPM Z+2. Significantly different
                  timing compared to AVR, AVRe, AVRe+.
AVRxt     A combination of AVRe+ and AVRxm. Available instructions are the same as AVRe+, but the
                  timing has been improved compared to AVR, AVRe, AVRe+ and AVRxm.
                  AVRrc AVRrc has only 16 registers in its register file (R31-R16), and the instruction
                  set is reduced. The timing is significantly different compared to the AVR, AVRe, AVRe+,
                  AVRxm and AVRxt. Refer to the instruction set summary for further details.

#include <avr/io.h>
#include <stdint.h>

int main (void) {
  DDRB = 1;                       // PB0 as an output
  TCCR0A = 1<<COM0A0 | 0<<WGM00;  // Toggle OC0A, CTC mode
  TCCR0B = 1<<WGM02 | 3<<CS00;    // CTC mode; use OCR0A; /64
  OCR0A = 15624;                  // 1 second; ie 0.5Hz
  while (1);
}

#include <avr/io.h>
#include <stdint.h>

int main (void) {
  DDRB = 1;                       // PB0 as an output
  // Set up ADC on PB2
  ADMUX = 1<<MUX0;                // ADC1 (PB1)
  ADCSRA = 1<<ADEN | 3<<ADPS0;    // Enable ADC, 125kHz clock
  // Set up waveform on PB0
  TCCR0A = 1<<COM0A0 | 3<<WGM00;  // Toggle OC0A, Fast PWM
  TCCR0B = 3<<WGM02 | 4<<CS00;    // Fast PWM with OCR0A as TOP; /256
  // Main loop
  for (;;) {
    ADCSRA = ADCSRA | 1<<ADSC;    // Start
    while (ADCSRA & 1<<ADSC);     // Wait while conversion in progress
    OCR0A = ADCL;                 // Copy result to frequency output
  }
}