108 lines
4.4 KiB
ReStructuredText
108 lines
4.4 KiB
ReStructuredText
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()
|
|
False
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
Further reading
|
|
---------------
|
|
|
|
For further information about using hardware interrupts see
|
|
:ref:`writing interrupt handlers <isr_rules>`.
|