How do you disable Int's in an interrupt

GSXRChris · Joined: 23 Mar 2005 Posts: 18

Hi all

How can I disable an interrupt in an interrupt service routine, the return at the end re-sets interrupts enabled

I want to do this:

DToolan · Posted: Sun Mar 27, 2005 5:58 am Post subject:

The code you have written does disable Timer1 interrupts.

When a hardware interrupt occurs, AND the "interrupt enable bit" for that interrupt is enabled, AND the "global interrupt enable bit" in the AVR status register is enabled..., then the "global interrupt enable bit" (also called the "I" flag) is cleared. This is done to prevent interruption of an ISR routine by some other system interrupt. You are correct that the Return at the end resets the I flag (this does not reset your Timer1 interrupt enable... only the global interrupt enable flag). Restoring the I flag (global interrupt enable) allows the handling of any interrupts that may have occured while busy in the isr... and if not, to be ready for some future interrupt. In your Timer1_isr, you have disabled the Timer1 interrupt. Timer1 will no longer be a source of future interrupts.

Are you trying to say that even though you have Timer1 interrupts disabled (in your isr) that you are still getting Timer1 interrupts after leaving the isr?

GSXRChris · Joined: 23 Mar 2005 Posts: 18

Thanks for the response DToolan

Yes I do get interrupts after I have disabled the interrupt in the isr, I had assumed that the interrupt mask was being retreived from the stack on return and that is why the disable interrupt was being ignored.

I thought that on entry to the isr a copy of the interrupt mask was pushed onto the stack and that the I flag was cleared, then on exiting the isr the interrupt mask was pushed back into the interrupt mask register and the I flag was set.

If this is not the case, could there be an unserviced interrupt in the the interrupt flag register (TIFR) that is fireing as soon as I re-enable interrupts the next time I sample timer1.

As I understand it there are two registers that are 'anded' to produce an interrupt, the interrupt mask (TIMSK) and the interrupt flag registers (TIFR) and as long as the I flag is set the interrupt will occur.

It says in the data sheet that if an interrupt is pending in the TIFR register it only gets cleared if the interrupt is serviced. It's possible that the Overflow flag is being set when in my INT0 isr, so even if I disable interrupts in that isr, next time I enable the timer the overflow flag that was set during the INT0 isr fires an overflow interrupt even though I zero the timer and disable interrupts. I've checked this by clearing the relevant TIFR bits to clear any pending Int0 and Timer1 overflow interrupts and it still does'nt work.

So I end up where I started thinking the Interrupt mask must be pushed and poped to and from the stack on isr return.

DToolan · Posted: Sun Mar 27, 2005 7:35 pm Post subject:

GSXRChris · Joined: 23 Mar 2005 Posts: 18

hi

No i dont got a JTAG Sad

The chip Mega162v

But I have sussed it out. It was pending interrupts causing my troubles. I checked that as I said by clearing the flags, or so I thought. What I hadnt noticed was that Bascom calls the GICR register the GIMSK register so I wasnt actually clearing the pending interrupts, funny thing was that the compiler didnt complain at me.

The code fragment was only an example to go on.
I need Timer0 running all the time, Int0 and Int1 sampled in a loop.
Here's the working isr:

DToolan · Posted: Sun Mar 27, 2005 8:45 pm Post subject:

GSXRChris · Joined: 23 Mar 2005 Posts: 18

Thanks for your help. Smile

I will stick with the gimsk dec for now. It works with Int1 isr operational as well. Change the prescaler on the timer and you can measure a wide range of input puleses. Very Happy

You can have timer0 free running for other tasks, timer3 too I suppose but Ive not checked that.

I posted the code so people wouldnt have to go over old ground. It's like everyone throwing up all the bricks when we only need to throw up one brick each.

Cheers
Chris