5. The Switch, callbacks and interrupts

The pyboard has 2 small switches, labelled USR and RST. The RST switch is a hard-reset switch, and if you press it then it restarts the pyboard from scratch, equivalent to turning the power off then back on.

The USR switch is for general use, and is controlled via a Switch object. To make a switch object do:

>>> sw = pyb.Switch()

Remember that you may need to type import pyb if you get an error that the name pyb does not exist.

With the switch object you can get its status:

>>> sw.value()

This will print False if the switch is not held, or True if it is held. Try holding the USR switch down while running the above command.

There is also a shorthand notation to get the switch status, by “calling” the switch object:

>>> sw()

5.1. Switch callbacks

The switch is a very simple object, but it does have one advanced feature: the sw.callback() function. The callback function sets up something to run when the switch is pressed, and uses an interrupt. It’s probably best to start with an example before understanding how interrupts work. Try running the following at the prompt:

>>> sw.callback(lambda:print('press!'))

This tells the switch to print press! each time the switch is pressed down. Go ahead and try it: press the USR switch and watch the output on your PC. Note that this print will interrupt anything you are typing, and is an example of an interrupt routine running asynchronously.

As another example try:

>>> sw.callback(lambda:pyb.LED(1).toggle())

This will toggle the red LED each time the switch is pressed. And it will even work while other code is running.

To disable the switch callback, pass None to the callback function:

>>> sw.callback(None)

You can pass any function (that takes zero arguments) to the switch callback. Above we used the lambda feature of Python to create an anonymous function on the fly. But we could equally do:

>>> def f():
...   pyb.LED(1).toggle()
>>> sw.callback(f)

This creates a function called f and assigns it to the switch callback. You can do things this way when your function is more complicated than a lambda will allow.

Note that your callback functions must not allocate any memory (for example they cannot create a tuple or list). Callback functions should be relatively simple. If you need to make a list, make it beforehand and store it in a global variable (or make it local and close over it). If you need to do a long, complicated calculation, then use the callback to set a flag which some other code then responds to.

5.2. Technical details of interrupts

Let’s step through the details of what is happening with the switch callback. When you register a function with sw.callback(), the switch sets up an external interrupt trigger (falling edge) on the pin that the switch is connected to. This means that the microcontroller will listen on the pin for any changes, and the following will occur:

  1. When the switch is pressed a change occurs on the pin (the pin goes from low to high), and the microcontroller registers this change.
  2. The microcontroller finishes executing the current machine instruction, stops execution, and saves its current state (pushes the registers on the stack). This has the effect of pausing any code, for example your running Python script.
  3. The microcontroller starts executing the special interrupt handler associated with the switch’s external trigger. This interrupt handler get the function that you registered with sw.callback() and executes it.
  4. Your callback function is executed until it finishes, returning control to the switch interrupt handler.
  5. The switch interrupt handler returns, and the microcontroller is notified that the interrupt has been dealt with.
  6. The microcontroller restores the state that it saved in step 2.
  7. Execution continues of the code that was running at the beginning. Apart from the pause, this code does not notice that it was interrupted.

The above sequence of events gets a bit more complicated when multiple interrupts occur at the same time. In that case, the interrupt with the highest priority goes first, then the others in order of their priority. The switch interrupt is set at the lowest priority.

5.3. Further reading

For further information about using hardware interrupts see writing interrupt handlers.