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    <p>You are facing a fundamental problem: an ISR is called with no data. In other words, the ISR function is not provided any data that indicates its source of the interrupt. </p> <p>The basic approach is that the person writing the code provides the linkage between input and action by hardcoding a function. The tricky approach you are looking for is a method by which the ISR can figure out what the source was.</p> <p>So you want to have N motors with 2N inputs from quadrature encoders. A single interrupt handler is attached to all the input pins on a CHANGE condition. The same handler gets called for all N motors. It can figure out which Motor to update by comparing the input pins to the values the last time it was called. If the input pins changed, then call that motor. Here is a psuedo code</p> <pre><code>Motor motor1; Motor motor2; onSomethingChanged() { static int in1aprev, in1bprev; static int in2aprev, in2bprev; int in1a, in1b; int in2a, in2b; in1a = digitalRead(.... same for other in's if( (in1a!=in1alast) || (in1b!=in1blast)) { motor1.encoderInput(in1a,in1b); in1alast = in1a; in1blast = in1b; } if( (in2a!=in2alast) || (in2b!=in1blast)) { motor2.encoderInput(in2a,in2b); in1a2ast = in2a; in1b2ast = in2b; } return; } </code></pre> <p>Not so long ago this type of function would be handled in an entire chip (see <a href="http://en.wikipedia.org/wiki/Intel_8259" rel="nofollow">programmable interrupt controller</a> ). The chip implements all the logic to trigger and capture the source of the interrupt. The main CPU just gets a trigger "something happened". The handler polls the chip to ask "what happened".</p> <p>Having offered this method, I'm not sure I would recommend. You cannot <strong>hide</strong> what is going on with your code. The motor must be physically wired to the correct pins. You have consumed a scarce resource -- you have to tell people.</p>
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    1. POOne interrupt function to modify a dynamically determined instance?
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    1. POOne interrupt function to modify a dynamically determined instance?
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    1. COYou exactly understood what I was looking for, thanks for the very informative answer. Clever solution, but in order to have the behavior I am looking for (add as many motors as the user wants) I need to write a lot of code which shouldn't have its place in an interrupt lasting less than 1ms. Worth a shot, I'm still happy to have brainstormed on it with you all.
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    2. COAgreed you definitely need to think about how long this might take as it directly impacts the encoder rate you can keep up with. If you are trying to run at 500 Hz encoder speed with only 50% time consumed by interrupt handler, then you have 1 millisecond. But that is 16000 cpu clock ticks...
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    3. COUpdate, it works nicely. The encoders call 2 interrupts every 1.8ms at full speed and it doesn't seem to bother it. Maybe because the content of the loop is "clocked": the maximum frequency of actions in it is 20Hz (motors rates feedback).
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