Pardon Me for Interrupting (Part Deux)

1/*
2 * Flash an LED two different ways. One with delay() and one with  timer interrupts.
3 * The interrupt driven LED is kept in sync using pin change  interrupts.
4 */
5
6#define LED 9
7#define BUTTON 7
8
9void setup()  {
10  Serial.begin(9600);
11  pinMode(LED_BUILTIN, OUTPUT);
12  digitalWrite(LED_BUILTIN,  HIGH);
13  pinMode(LED, OUTPUT);
14  pinMode(BUTTON, INPUT_PULLUP);
15  
16  // Disable interrupts while things are being set up.
17  cli();
18
19  // Set  up Timer/Counter Control Registers. See ATmega328P data sheet section 15.11 (Remember,  bits are numbered from zero.)
20  TCCR1A = B00000000;  // All bits zero for "normal"  port operation. 
21  TCCR1B = B00001100;  // Bit 3 is Clear Timer on Compare match  (CTC) and bit 2 specifies a divide by 256 prescaler.
22  TIMSK1 = B00000010;  //  Bit 1 to raise an interrupt on timer Output Compare Register A (OCR1A) match.
23  OCR1A = 62500;       // Counter compare match value. 16MHz / prescaler * delay  time (in seconds.)
24   
25  // Set up Pin Change Interrupt Control Registers  to detect incoming transitions.
26  PCICR = B00000100;   // Enable only PCINT2  (PCINT23..16 or Arduino pins D0..D7).
27  PCMSK2 = B10000000;  // Enable only INT23  (Arduino D7 pin).
28
29  // Reenable the interrupts 
30  sei();
31}
32
33void  loop() {
34  digitalWrite(LED, LOW);
35  delay(1000);
36  digitalWrite(LED, HIGH);
37  delay(1000);
38}
39
40// Interrupt handler for timer match.
41ISR(TIMER1_COMPA_vect)
42{
43  // Access pin 13 directly performing an XOR on Port B (digital pins 8 - 13).
44  // Same as "digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN))"
45  PORTB  ^= B00100000;  // Toggle bit 5, which maps to pin13.
46}
47
48// Interrupt handler  for pin change.
49ISR(PCINT2_vect)
50{
51  // To ensure the incoming bit is sampled  in the middle for a more accurate reading, the timer counter
52  // should be at  the middle of its maximum value when the pin change happens. The digital counter  is
53  // extremely accurate, so speeding or slowing its frequency is not needed  so much as simply ensuring
54  // the counter is at 1/2 its maximum value when  the pin change interrupt occurs.
55  TCNT1 = OCR1A >> 1; 
56}