Adding Prioritization to an Single Level Interrupt Priority System

Summary of technique: Add a software structure that executes only the highest priority pending interrupt within the ISR polling loop. Then start again at the top of the polling loop instead of polling all possible ISRs. This gives you a prioritized non-preemptive interrupt service routine scheduler.

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With some microcontrollers, all of your interrupts come in at the same priority level (for example, via an external interrupt request pin). The usual thing to do in that case is create a polling loop to check all the sources of interrupts and see which one needs to be serviced by looking at peripheral status registers.  For example:
if(HWTimerTick)  { ... ISR to service hardware timer tick ... }
if(ADCReady)  { ... ISR to service A to D converter ... }
if(SerialPortDataInReady ) { ... ISR to read a serial port byte... }
if(SerialPortDataOutReady) { ... ISR to write a serial port byte ... }
...
(Of course this isn't real code ... I'm just sketching a flow that you've seen before if you've written this type of ISR that polls all the devices that can cause interrupts to see which one actually needs to be serviced.)

If only one of these devices is active, then this approach should work pretty well. And if you do system-level testing probably things will work fine -- at least most of the time.

But the way you can get into trouble is if one of the interrupts has a short deadline for being serviced. Let's say you have the above code and are seeing serial input bytes being dropped once in a while.  What could be happening?

One cause of dropping bytes might be that the HW Timer Tick and/or the ADC Ready interrupts are active at the same time that the serial port data input interrupt is ready. You need to execute them before you can get data from the serial port. If the sum of their two execution times is longer than the time between serial byte arrivals, you're going to take too long to get to the serial port input ISR and will drop bytes.

You might buy a faster processor (which might be unnecessary as we'll see), but before doing that you might reorganize the code to put the serial input first in the list of ISRs so you can get to it faster when an interrupt comes in:
if(SerialPortDataInReady ) { ... read a serial port byte... }
if(HWTimerTick)  { ... service hardware timer tick ... }
if(ADCReady)  { ... service A to D converter ... }
if(SerialPortDataOutReady) { ... write a serial

And that will *almost* work. Things might get a little better, but it won't cure the problem. (Or, MUCH worse, it will cure the problem in testing only to have the problem reappear in the field after you've shipped a lot of systems!)  Now when you get an interrupt you'll service the serial port input ISR first. But, then you'll go off and do the other ISRs. If those other ISRs take enough time, you will be stuck in those other ISRs too long and will miss the next byte -- you won't get back to the top of the list of ISRs in time.

You might try re-enabling interrupts inside any long ISRs to let the serial port get processed sooner. But resist the temptation -- that probably won't work, and will likely result in stack overflows due to recursive interrupt processing. (Simple rule: NEVER re-enable interrupts from inside an ISR.)

What we really need here is prioritization. And it's pretty easy to get even though we don't have hardware interrupt prioritization. All you have to do is (1) put the checks for each ISR in priority order, and (2) only execute the first one in the list each time you process interrupts. This can be done as follows:
if(SerialPortDataInReady ) { ... read a serial port byte... }
else if(HWTimerTick)  { ... service hardware timer tick ... }
else if(ADCReady)  { ... service A to D converter ... }
else if(SerialPortDataOutReady) { ... write a serial port byte ... }

Now only the first active interrupt will be serviced and the rest ignored. When you drop out of this structure and exit, any pending interrupt will re-trigger the checks from the beginning, again executing the highest priority interrupt that is still active (i.e., the first active one in the list). This will continue until all pending interrupts have been processed. You can use a "while" loop around the code above, or in many systems it may make sense just to exit interrupt processing and let the hardware interrupts re-trigger to re-run the polling code as a new interrupt.

This approach means that the worst case delay between processing serial input bytes is no longer all the ISRs running (if all interrupts are active). Rather, the worst case is the single longest ISR happens to be running, completes, and the serial port input ISR runs next. This happens because the list only runs at most one ISR rather than all of them. If that one ISR runs too long to meet deadlines, then it's probably too "fat" and should be simplified or its job moved out of ISRs and into the main loop.

There is no free lunch. The lowest priority ISR (the one at the end of the list) might starve. Making sure you meet all your ISR deadlines is trickier with this structure. Without the "elseif" approach the worst case timing is easy to compute -- it is the run time of all ISRs. But it might be too slow to live with. With this structure you have a nonpreemptive prioritized scheduling system for ISRs, and need to use suitable math and a suitable scheduling approach. Generally you'd want to use rate monotonic analysis (RMA) suitably adapted for the ISRs being non-preemptive. The analysis may be a little more complex, but this approach might help you salvage a situation in which you're missing deadlines and have already committed to a certain speed of microcontroller.

(Note on terminology: technically the whole thing is one big ISR that calls a different function depending upon what's active. But I'm calling each such function an ISR because that is really what it does ... you're using a software dispatcher to pick which ISR to run instead hardware prioritization logic to pick an ISR.)