Blinking a led at 38kHz on an attiny10. Gaining extra precision by swithching from the default :8 prescaler to the :1 prescaler (8MHz clock)

blink_38khz_1_prescaler.c

#include <avr/interrupt.h>        // cli

int main (void) {
  char oldsreg = SREG;            // save the interrupt setting register to oldsreg
  cli();                          // disable all the interrupts (SREG = 0)
  CCP = 0xD8;                     // signature to CCP
  CLKMSR = 0;                     // use clock 00: Calibrated Internal 8 MHzOscillator
  CCP = 0xD8;                     // signature
  CLKPSR = 0;                     // set prescaler to :1 (0x00)
  SREG = oldsreg;                 // restore the sreg, enabling the interrupts

  DDRB = 1;                       // set DDRB.PB0 to output
  OCR0A = 105;                    // set OCR0A
  TCCR0A = 1<<COM0A0;             // set timer to toggle PB0 on compare match
  TCCR0B = 1<<WGM02 | 1<<CS00;    // set CTC mode | enable and set prescaler :1
  while(1);
}

That's a dangerously lightweight program. I might be able to optimize it further into AVR assembly, especially if the halt instruction leaves the clock running.
Although, to be honest, I'm not really sure it would be useful. I'm going under the assumption that OCR0A contains the number of clock cycles needed to toggle PB0. I imagine it would be relatively easy to drop one of the writes to CCP, like so:

LDI r0, 0xD8
LDI r1, 0x00 ;Might not be necessary
OUT CCP, r0
OUT CLKMSR, r1
OUT CLKPSR, r1

could replace four lines in your C++ program:

CCP = 0xD8;
CLKMSR = 0;
CCP = 0xD8;
CLKPSR = 0;