Pardon Me for Interrupting

1/*
2 * Flash an LED two different ways. One with delay() and one with  timer interrupts.
3 */
4
5#define LED 9
6
7void setup() {
8  
9  pinMode(LED_BUILTIN,  OUTPUT);
10  pinMode(LED, OUTPUT);
11
12  // Disable interrupts while things  are being set up.
13  cli();
14
15  // Set up Timer Control Registers. See ATmega328P  data sheet section 15.11 (Remember, bits are numbered from zero.)
16  TCCR1A =  B00000000;  // All bits zero for "normal" port operation. 
17  TCCR1B = B00001100;  // Bit 3 is Clear Timer on Compare match (CTC) and bits 0..2 specifies a divide  by 256 prescaler.
18  TIMSK1 = B00000010;  // Bit 1 will raise an interrupt on  timer Output Compare Register A (OCR1A) match.
19  OCR1A = 31250;       // Counter  compare match value. 16MHz / prescaler * delay time (in seconds.)
20
21  // Reenable  the interrupts 
22  sei();
23}
24
25void loop() {
26  digitalWrite(LED, LOW);
27  delay(500);
28  digitalWrite(LED, HIGH);
29  delay(500);
30}
31
32ISR(TIMER1_COMPA_vect)
33{
34  // Access pin 13 directly performing an exlusive or on Port B (digital pins 8  - 13).
35  // Same as "digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN))"
36  PORTB ^= B00100000;  // Toggle bit 5, which maps to pin13.
37}

1/*
2 * Flash an LED two different ways. One with delay() and one with
3  timer interrupts.
4 */
5
6#define LED 9
7
8void setup() {
9  
10  pinMode(LED_BUILTIN,
11  OUTPUT);
12  pinMode(LED, OUTPUT);
13
14  // Disable interrupts while things
15  are being set up.
16  cli();
17
18  // Set up Timer Control Registers. See ATmega328P
19  data sheet section 15.11 (Remember, bits are numbered from zero.)
20  TCCR1A =
21  B00000000;  // All bits zero for "normal" port operation. 
22  TCCR1B = B00001100;
23  // Bit 3 is Clear Timer on Compare match (CTC) and bits 0..2 specifies a divide
24  by 256 prescaler.
25  TIMSK1 = B00000010;  // Bit 1 will raise an interrupt on
26  timer Output Compare Register A (OCR1A) match.
27  OCR1A = 31250;       // Counter
28  compare match value. 16MHz / prescaler * delay time (in seconds.)
29
30  // Reenable
31  the interrupts 
32  sei();
33}
34
35void loop() {
36  digitalWrite(LED, LOW);
37
38  delay(500);
39  digitalWrite(LED, HIGH);
40  delay(500);
41}
42
43ISR(TIMER1_COMPA_vect)
44{
45
46  // Access pin 13 directly performing an exlusive or on Port B (digital pins 8
47  - 13).
48  // Same as "digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN))"
49
50  PORTB ^= B00100000;  // Toggle bit 5, which maps to pin13.
51}