I have doubt that micro controller will support multiple interrupt routines for a single timer.
For example, single timer which interrupts at 5ms, 10ms, 1s etc. In my design I am using 3 timers: one timer interrupts every 1ms for some status check, other two at 50ms and 1s.
I am curious to know about why can't I have controller which has more than one ISR and also, instead of interrupts at timer overflow, why can't it interrupt at its period match.
There may be some that have this capability, but most don't. You will need to maintain a count of cycles elapsed and delegate to the appropriate routine from there.
No, but there are plenty of microcontrollers with multiple timers, each run of which can throw interrupts at different intervals.
Of course, each timer will need its own interrupt routine. It doesn't seem like you would want to treat your 5ms interrupt the same as a 1s interrupt, but if you were dead set on that type of architecture, all the interrupt routines can have the same code in them. That, of course, would use up some extra program memory, even if you just have all the interrupts call the same subroutine.
I can only speak for AVR's but I know the one's I used only had 2-3 timers. But I don't see a need for using multiple timers. You can just use one where the timer's prescaler and compare value are configured where a compare match ISR is thrown at some known time interval (every 1ms for example). You can then keep track of the time and have a switch statement to control what action you want done.
Timer ISR with variable period is fairly easy to achieve.
Variant 1. Timers usually have a countdown register, which can be updated by the code. The countdown initial value can be updated inside the ISR itself on a case by case basis. A state machine can be used to decide which initial value to assign to the counter.
Variant 2. Another option is to set up the timer for the shortest duration then count interrupts if you need longer duration. For example, you need to do 3 different things every 5ms, 10ms, 1s. Set the timer for 5ms, within the ISR increment a counter** every time ISR is called. In the main() loop, compare the counter to thresholds. For 10ms duration the threshold would be 2, for 1s the threshold would be 200. Once the counter is equal to the threshold, the main() does what needs to be done at that interval.
A good thing about this approach is that you can have one timer and one ISR, but multiple thresholds (multiple counters too, if needed). The variable timing would be implemented in the code, rather than in the hardware timer.
Note that, this approach requires a main() loop, which always loops faster than the shortest interval. Accuracy of the timing should also be considered. If that becomes a problem, there are variations of this pattern, which can work with a slower main() loop.
** counter and flags are global variables visible to both ISR and main()
