Electronic – Interrupt Service Routines and I2c together

i2cinterruptspic

I would like to ask a question that has been on my head for a while

I have done coding small ISR's for pic and AVR controllers and i have successfully tested them on board.And i have just learned some days ago that you need to keep ISR short.

So I have this ISR that runs each 2 ms from timer.

Well the doubt I have is If I have an I2c communication running parallel and then the interrupt occurs, What will happen to the I2C communication.

Will the data transmission gets damaged?.

If The ISR takes 2ms to execute, what will happen to the I2C communication meanwhile.

Can it cope with such delays? Or is it my lack of understanding.

Best Answer

The processor won't execute any foreground code during the interrupt. If the IIC is strictly handled in hardware, then that won't matter. If this processor is the IIC master, then it won't matter either since you own the clock and can go as slow as you want (most slaves allow this).

However, a interrupt routine taking 2 ms with other things going on, like IIC communication, at the same time sounds like bad overall architecture. The right answer is to step back and understand what all the processor needs to accomplish, what limited set of things really are low-latency that it must react to, and re-architect the firmware from there. Propagating a bad design by working around previous mistakes just makes things even worse.

Related Solutions

PIC interrupt delay_ms

Instead of using delay_ms() to do your LED timing, you should use the timer interrupt for that. I don't know MikroC at all, but make sure you have the timing interrupts correct first. Then, figure out the duration between two successive timer interrupts. Using this value you can then add a global (static) variable to hold your upper bound for your counter (I guess in your case, cnt). You would calculate these two upper bound values manually: one for mode one, one for mode two (i.e. your counter should count up and reset back to 0 once it reaches the respective value). These values will determine your 3 sec or 1 sec delay. You could have another global variable to determine whether the LED should be on or off but you could do without it.

Now in the main code, you just need to change your counter bounds inside your if-else statement.

Electronic – Interrupt Service Routine in C – function at specific address

You'll find the information you need in the compiler's manual: MPLAB C18 C COMPILER USER'S GUIDE.

From page 37 of the user's guide:

MPLAB C18 does not automatically place an ISR at the interrupt vector. Commonly, a GOTO instruction is placed at the interrupt vector for transferring control to the ISR proper. For example:

void lowPriorityISR(void);

#pragma code low_vector=0x18
void interrupt_at_low_vector(void) { 
    _asm GOTO lowPriorityISR _endasm 
}
#pragma code

#pragma interruptlow lowPriorityISR
void lowPriorityISR() {
}

When implementing a high priority ISR, you cannot use the interrupthigh pragma as you would expect. You'll have to use the interrupt pragma instead, as is described here.

If you can change the compiler: using interrupts is easier in the Microchip XC8 or HI-TECH PICC compilers.

Best Answer

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PIC interrupt delay_ms

Electronic – Interrupt Service Routine in C – function at specific address

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