https://itp.nyu.edu/classes/prototypingelectronicdevices/wp-content/uploads/sites/105/2022/10/Class-9-Multitasking.pdf

<aside>

What exactly is delay() doing in Arduino? in ArduinoCore-avr Github page, the definition is as below:

void delay(unsigned long ms)
{
	uint32_t start = micros();

	while (ms > 0) {
		yield();
		while ( ms > 0 && (micros() - start) >= 1000) {
			ms--;
			start += 1000;
		}
	}
}

Ways to utilize the delay time —> use millis() to replace delay().

</aside>

<aside>

• if we look into the structure and definition of avr, searching for avr gcc, we see that the size for “long” is 4 bytes (32 bits)

  

• Arduino millis() and micros() are both defined as “long”, see the below definition in Arduino documentation:

  unsigned long millis()
  {
  	unsigned long m;
  	uint8_t oldSREG = SREG;
  
  	// disable interrupts while we read timer0_millis or we might get an
  	// inconsistent value (e.g. in the middle of a write to timer0_millis)
  	cli();
  	m = timer0_millis;
  	SREG = oldSREG;
  
  	return m;
  }
  
  unsigned long micros() {
  	unsigned long m;
  	uint8_t oldSREG = SREG, t;
  	
  	cli();
  	m = timer0_overflow_count;
  #if defined(TCNT0)
  	t = TCNT0;
  #elif defined(TCNT0L)
  	t = TCNT0L;
  #else
  	#error TIMER 0 not defined
  #endif
  
  #ifdef TIFR0
  	if ((TIFR0 & _BV(TOV0)) && (t < 255))
  		m++;
  #else
  	if ((TIFR & _BV(TOV0)) && (t < 255))
  		m++;
  #endif
  
  	SREG = oldSREG;
  	
  	return ((m << 8) + t) * (64 / clockCyclesPerMicrosecond());
  }
  
  

• millis() can store about 50 days of data before overflow: 2^32/1000/60/60/24=49.71

• micros() can store about 1 hour of data before overflow: 2^32/1000/1000/60/60=1.193 </aside>

<aside>

Interrupt

• In the process of the Arduino code running, “Interrupt” happens whenever certain conditions are met, it stops the running code for a short amount of time, so that some “task” can be done in the background, ex: keep track of time (increase the millis), external interrupt
• Time Track

void init(){

}

• External Interrupt this function can be utilized when there’s a time consuming and un-breakdownable function in the code

  delay(1000);
  checkButtonPressed();
  

  During the 1 second of delay, if I still want to use the checkButtonPressed(), and detect the state of the button, I have to use external interrupt, so that when certain condition is met (in this case, button is pressed), the delay() would be postponed for just a small amount of time to mark the button as pressed. For Uno, this function only exists in pin 2/3.

  Real time usage: a button check during WIFI connection (button check is quick; WIFI connection takes a lot of time)

  However, if the data that is being read when external interrupt happens is more than 1 byte(Uno can read only 1 byte at a time), if the interrupt happens directly between reading different bytes, the data will be incorrect. —> the simplest way is to use volatile and atomic to avoid this.

• Internal ways of stopping external interrupt in Arduino logic:

  As the code comment, the cli() will disable all interrupts when reading time, so data is precise.

</aside>

unsigned long millis()
{
	unsigned long m;
	uint8_t oldSREG = SREG;

	// disable interrupts while we read timer0_millis or we might get an
	// inconsistent value (e.g. in the middle of a write to timer0_millis)
	cli();
	m = timer0_millis;
	SREG = oldSREG;

	return m;
}

<aside>

question:

I want to know about how to use documentation for Arduino/pi on GitHub. What info can we use?

</aside>