Normal code runs in a Task context, that is to say there are no particular restrictions on what functions can be called. Sming is a single-threaded framework, so all tasks execute sequentially.

However, there are some rules you need to follow when writing code which runs in an interrupt context:

  • Use IRAM_ATTR attribute on the interrupt handler
  • Don’t call any code which isn’t in IRAM, or marked with __forceinline. Note: inline by itself is just a compiler ‘hint’ and there is no guarantee the code will actually be inlined.
  • Keep the code as brief as possible
  • Don’t use the heap (malloc, free, new, etc.). Buffers, etc. must be pre-allocated in task context.
  • Put processing code into a separate function and add to the task queue SystemClass::queueCallback(), or use a SimpleTimer or Timer.
  • Be wary if using Delegate callbacks as the compiler may need to use the heap, if parameter lists do not match. For this reason, avoid capturing lambdas and method callbacks. If in doubt, stick with regular ‘C’ function pointers as defined by the InterruptCallback type.

Contact bounce

If you use a jumper wire to ground the input pin (or even a proper switch) then it will bounce around as contact is made and then broken. This means you’ll get multiple outputs instead of a clean signal, which will cause havoc with interrupt handlers.

One solution is to use a ‘debounce circuit’, which can be as simple as this:

               ___ 100nF
INPUT   >--------------> GPIO PIN

Provided pull-ups are enabled on the GPIO, this is sufficient to slow the input.

An alternative solution is to poll the input instead. See Tasks.