Electronic – Interrupts and timing applications

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While learning about multiplexing 7 segments using interrupts in PIC, it was found that when this was used in real world timing applications, the frequent ISR causes a step by step cumulative delay in the main loop, which then increases further from the real world time, owing to the delay caused by the main program by ISR subroutine servicing.

I use 20 MHz crystal running PIC with timer0 for refreshing multiplexed displays but the delay caused by ISR is evident in the main loop of the program when a real-time measurement is done by a delay routine etc. Thus when 5 minutes are counted, it adds some seconds depending on the timer0 setting for ISR. Like 5 sec for 5 min and a minute or so for 1 hour etc.

Is there any way to solve this apart from using serial displays ??

Best Answer

Instead of using a main loop delay to compute the time, which gets disrupted by the hardware counter, use the hardware counter itself to provide the timing.

Set the timer0 to interrupt (say) every 1mS. This becomes your system's tick, or heartbeat. Use the interrupt routine to increment the mS counter, and to refresh the displays.

In the main loop, use the mS counter as your 'wall clock', wait until it has advanced 300,000 ticks, and you have waited 5 minutes within a non-cumulative accuracy of 1mS.

This answer is intentionally incomplete, there are some more technical details that may be necessary depending on the length of your time check routine, but it would overload the answer to present a treatise now. Once you start getting into the details, you'll find that getting clean communication between the main loop and ISRs involves a bit of care, declaring variables volatile, taking snapshots, disabling interrupts for critical periods, but it is all documented and it's the way everybody else does it.

Related Solutions

Electronic – Interrupts with 16f887 and MPLAB

Try clearing INTEDG bit of OPTION_REG register. From the datasheet page 30(32):

INTEDG: Interrupt Edge Select bit
        1 = Interrupt on rising edge of INT pin
        0 = Interrupt on falling edge of INT pin

When PORTD is high, does the LED turn ON or OFF?

Also check out switch contact bounce. Here is a link on it and how to debounce. To check it is the contact bounce causing the thing that is seen as a "problem", add a delay of 500msec or 1 sec after PORTD=0xff; in the interrupt routine. If the port is high, your problem is contact bounce if the above suggested solution doesn't solve your problem -that is clearing INTEDG bit.

You don't need that GIE=1; in the last line of the interrupt block.

I have no experience with Hi-Tech C compiler. Chances are low but try doing INTE=1; before GIE=1; in the main function.

Edit

Try changing void intMain() to void interrupt intMain(). According to

HI-TECH C® for PIC10/12/16 User’s Guide 3.8 INTERRUPT HANDLING IN C (Page 86) Link

Edit After Second Question

According to the same manual mentioned above page(299):

To produce an infinite loop, use for(;;).

You are trying to assign an 'integer' value to a 'character' variable as I quoted below. Define variable i as unsigned char. Compiler should take care of this, but it is worth trying.

...
int i,k;
...
PORTD=i;
...

Electronic – Using multiple external interrupts in PIC

This is C pseudo-code to explain one idea. It uses and exclusive OR to work out which pins have changed and will call your different handlers within the one RBIE interrupt. Depending on how critical the application is you may want to check how the PIC handles situations such as a port changing while the interrupt is executing to make sure you won't miss any events.

int old_port_b;

void isr_handler()
{
    int new_port_b, changed_pins;
    new_port_b = read_port_b();
    changed_pins = new_port_b ^ old_port_b;
    if (changed_pins | 1)
        rb0_hander();
    if (changed_pins | 2)
        rb1_hander();
        // ... etc
    old_port_b = new_port_b;
}

int main()
{
    old_port_b = read_port_b();
    enable_interrupt();
}

Best Answer

Related Solutions

Electronic – Interrupts with 16f887 and MPLAB

Electronic – Using multiple external interrupts in PIC

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