[Icc-avr] Global Interrupts

[John Baraclough]

Andrew,

Firstly when you change the subject in a post please delete the 
preceeding irrelevent stuff, so that we can identify your real question.

It depends on how long your ADC handling function takes. and how you 
have set up the timer. A timer interrupt will remain pending until the 
ISR is executed, and it is the ISR execution which will clear the 
interrupt flag. Now, if the ADC handler takes longer than the interval 
between timer interrupts then you will miss an interrupt and the timing 
will be incorrect. Also, if you have set up the timer so that it 
requires the ISR to reload the timer counter or change the compare 
register, then you will have errors from the additional latency. To be 
100% bomb-proof with timer interrupts they must be set up so the timer 
reloads or resets in hardware to avoid latency issues and the interval 
between timer interrupts should be considerably longer than the slowest 
non-interruptible code in your main program flow.

John




Andrew wrote:
> Hi,
>  
> i have an ADC connect to my  system using SPI and in the back ground a 
> tick timer overflow interrupt (100ms). when i call my read ADC 
> function to avoid any interrupts messing things i was going to do the 
> follwing: -
>  
>
> CLI(); // Disable all interrupts
> Get 4 Channels of results
>
> SEI(); // Re-enable interrupts
>
>  
>
> My question is will doing that screw the timer up or will it carry on 
> from where it left off after i re-enable the interrupts.
>
>  
>
> Andy
>