// Make clock source for SA1000 board
// Uses
// https://github.com/SpenceKonde/ATTinyCore
// Tools options selected
// Board ATtiny 25/45/85 (No bootloader)
// Chip ATtiny45
// Clock source 18.432 MHz externa or 9.216 MHz
// Timer 1 CPU frequency
// LTO (Link time optimization) enabled
// mills/micros enabled
// EEPROM retained
// BOD (brown out detection) 4.3V. 
// If using a 3.3V board to program the chip will program but not run due to BOD
// Programmed using Arduino as ISP.
// Used these instructions to wire up programmer. Also wired crystal and 18pf caps to crystal pins
// https://www.electronics-lab.com/project/how-to-program-attiny13attiny13a-using-arduino-ide/
// If you don't have the crystal attached after you program bootloader you can't talk to the chip
// until you attach a crystal.
// Note that most instructions say use pins 10-13. Current code default is 10, MOSI, MISO, and SCK.
// If you wired to 10-13 you need to define USE_OLD_STYLE_WIRING in ArduinoISP sketch
// Need to switch to board you are using to load the programmer sketch and then
// then switch to ATtiny board to tools->burn bootloader then program 
// sketch->upload using programmer
// Sometimes need to burn bootloader twice for it to work.
//
// PB0 is output clock ckip pin 5
//
// Uplaod sketch with upload using programmer.

// Target frequency 271250. With 18.432 MHz clock divide toggle every 34 clock 
// gives 271059 Hz or 0.07% slow. OEM manual says clock needs to be within 0.1%
// so good enough.

// **************************************************************
// Set this to 34 for 18.432 MHz oscillator. Use 17 for 9.216 MHz
unsigned int loop_count_pll = 17;
  

void setup() {
  // put your setup code here, to run once:
  pinMode(PB0, OUTPUT);

  TCCR0B = B00000000;// Stop Timer/Counter0 clock by setting the clock source to none.
  TCCR0A = B00000000;// Set Timer/Counter0 to normal mode.
  TCNT0  = 0;// Set Timer/Counter0 to 0
  OCR0A = loop_count_pll-1;// Set the Output Compare for Timer/Counter0
  TCCR0A = B01000010;// Set Timer/Counter0 to CTC mode. Set OC0A to toggle.
  TCCR0B = B00000001;// Start Timer/Counter0 clock by setting prescale of PLL source.
}

void loop() {
  // put your main code here, to run repeatedly:

}