Micros rollover in ISR

Using Arduino Programming Questions
1

Hi. I have a situation with a non-working button in my sketch. From debugging print I think this is related to micros rollover, but I don't know how to change/fix this.

When running the sketch, buttons work ok at start. Debugging info shows that variable last_micros increases after each button push, as it should. But after running for 71(?) minutes the micros() rollover, and that value is now lower than last_micros. And no button push is recognized.

I am using: Mega2560, IDE 1.0.2. When I search this forum, I have only found statements that supports the existing code. (But it don't work, at least for me.)

Copy of some of the code:

volatile int push[2] = {0,0};        // Variable to pass button press back to main loop
long debouncing_time = 40;           // Debouncing Time in Milliseconds
volatile unsigned long last_micros;  // Variable to keep time while debounce
ISR(PCINT0_vect) // handle pin change interrupt for PCINT0 to PCINT7 here, pin D53-D50 and D10-D13
{
  static uint8_t btn = _BV(PB5) | _BV(PB6);
  uint8_t t = PINB;  // read port status

  // check if buttons are pushed
  t &= _BV(PB6) | _BV(PB5); // D11 D12
  if (t != btn) {  // activity on pins, we're only interested in high to low transitions
    if((long)(micros() - last_micros) >= debouncing_time * 1000) {  
      push[0] |= !(t & _BV(PB5));
      push[1] |= !(t & _BV(PB6));
      btn = t;
      last_micros = micros();  // start/restart bounce guard timer (covers make and break)
    }
 }  
}
void loop() {
.
.
.
  // do data logging if it is time for it
  if(millis() - logTime>LOG_DELAY) {
    logTime += LOG_DELAY;
    Serial.print("brewController loop, do dataLogger");
    Serial.print(" micros:");
    Serial.print(micros());
    Serial.print(" last_micros:");
    Serial.print(last_micros);
    dataLogger();
  }
}

The debugging print is in a loop that is activated every 2.nd second.

2

Why are you casting your micros calculation to long? You want to keep it as unsigned long so it works properly.

3 
volatile int push[2] = {0,0};        // Variable to pass button press back to main loop
...
      push[0] |= !(t & _BV(PB5));
      push[1] |= !(t & _BV(PB6));

When does push ever get un-set?

4

@majenko:
I copied this part of the code. Can you explain how removing (long) would make a difference?

@Nick:
Push variable is reset in the main loop. It is used together with the MENWIZ library. Sorry if this was confusing.

void loop() {
.
.
  if ( push [0] > 0 ) { // confirm
    virtualButton = 6;
    push[0] = 0;
  }
  if ( push [1] > 0 ) { // escape
    virtualButton = 5;
    push[1] = 0;
  }
.
.
  // do data logging if it is time for it
  if(millis() - logTime>LOG_DELAY) {
    logTime += LOG_DELAY;
    Serial.print("brewController loop, do dataLogger");
    Serial.print(" micros:");
    Serial.print(micros());
    Serial.print(" last_micros:");
    Serial.print(last_micros);
    dataLogger();
  }
}
5

Take the following snippet:

    unsigned long a, b;

    a = 1000304UL;
    b = 1008UL;

    printf("%lu\n", b - a);
    printf("%ld\n", (long)(b - a));

b - a gives 4293968000, which is greater than whatever threshold you like - say 1000. That deals with your rollover nicely.
Cast that to a signed long, and you get: -999296 - that's not going to be above your threshold. Rollover broken.

6

It works! Thank you!