Working with state in MicroPython Timer callbacks

MicroPython supports hardware and software timers with callbacks. Timers can be configured as single-shot or periodic events meaning they trigger only once or on a periodic basis. You can bind a Python function to the timer running in either mode.  Your function will receive a callback whenever the timer rolls over or expires. 

Hardware timers are bound to the actual CPU timers and typically correlate 1:1 to hardware devices.  This means the hardware timer configuration is hardware specific to the size and capabilities of the hardware timers.  In general, you can only tie one callback to each timer because the callback is bound to the hardware timer interrupt handler.  

MicroPython also supports software timers. Software timers are the only option on CPUs like the ESP8266 where hardware timers are scarce or are dedicated to other functions. They have the same callback/handler restrictions as hardware timers.

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Initializing a Timer

Timers are started with an init() call.  You can specify the timer behavior and callback function as part of the init() call.  The callback function() will be invoked at the end of every timer period.

Timer.init(*, ..., callback, ...)

Some timers also have an API that lets you set or change just the callback function with a Timer. callback(callback_func). Timer APIs may vary slightly by IoT device type so check the documentation for your device.

Callback Parameters

MicroPython timer callbacks only include a reference to the timer itself.  No callback-specific data other than the timer reference is made available as part of the invocation. 

def callback_handler(t):

This means that using the exact same bare function as a callback across timers has no way of maintaining different states for each timer. Ex: Each timer is bound to different I/O pins.   

A better alternative is to have separate handler instances for each timer.  An Object-Oriented pattern for this is described below.

Timer rollover callbacks execute as an interrupt handler

No memory should be allocated as part of the callback function invocation. 

Timer callbacks are invoked by interrupt handlers while the primary Python thread is idle. This is probably the reason that the MicroPython timer callback signature is different from the Thread.Timer callback signature in regular Python. 

You are restricted in what you can do in an interrupt handler with respect to memory allocation. This is called out on many of the MicroPython hardware-specific timer documentation pages. Ex: pyb timer callback docs

Note: Memory can't be allocated during a callback (an interrupt) and so exceptions raised within a callback don't give much information

Make sure that you have pre-allocated any memory required for callbacks.  The I/O pin toggling callback example shown below allocates no memory when invoked.

  • Keep the code as short and simple as possible.Avoid memory allocation: no appending to lists or insertion into dictionaries, no floating point.
  • Consider using micropython.schedule to work around the above constraint.
  • Where an ISR returns multiple bytes use a pre-allocated bytearray. If multiple integers are to be shared between an ISR and the main program consider an array (array.array).
  • Where data is shared between the main program and an ISR, consider disabling interrupts prior to accessing the data in the main program and re-enabling them immediately afterwards (see Critical Sections).
  • Allocate an emergency exception buffer (see below).
  • ISR’s cannot create instances of Python objects. 

Example Flow

This describes the setup and execution flow for a Timer driven callback in . 
  1. Create a web server that has a reference to the timer and a callback function
  2. The web server responds to requests to start and stop the timed behavior which results in init()/deinit() calls for the timer
  3. When running, the Timer invokes the callback function whenever the timer period is exhausted.  The callback in this case just toggles the state of an LED.
The example can start and stop a timer at will by invoking init()/deinit().  The timer invokes the callback function provided in the init() call whenever a timer period expires. The callback function processes whatever it needs to do.   Periodic timers then reset the period count setting up the next callback.  

Note: We can change the callback function bound to the timer every time we init() it.  

This entire process happens essentially as background work while the web server in the example is idle waiting for requests.

Holding State in Global Variables

Callback functions() only receive the timer itself as a parameter.  They can't allocate memory and they don't get passed in any other state.  It is best to write callback functions so that they don't require any persistent state or state across calls but this is not always possible.  Ex:  you wish to send an MQTT message to the cloud on a periodic basis.  You would need the connection information for the target URL.

Any required persistent state must be allocated outside of the function definition.  The function definition must be able to reference that external memory. We don't know what thread the function will run in and should assume that it may be in a different context than the primary execution loop. 

The example below declares a variable periodic_target I/O pin outside the function so that it maintains its state across callbacks.  The variable is described as global inside the function so that the code knows to use the externally declared value instead of creating one that exists only for the function invocation.

periodic_target = Pin(2, Pin.OUT)

def toggle_pin_callback(t):
    """sutable for call from a timer event"""
    global periodic_target
    periodic_target.value(not periodic_target.value())

Global variables are generally frowned upon and can create unexpected coupling.  They are also a problem if we wish to use the same function for multiple timers. Ex: flashing different LEDs at different rates bound to timers with different periods.

The function described above can be passed as a callback by reference.  The line below shows the function being passed in as a callback handler.

    a_periodic_operator = PeriodicOperator(Timer(-1), 500, toggle_pin_callback)

Holding State in an Object

We need a reliable way of providing persistent storage to the event handler across invocations. Python supports object-oriented programming.   We can define an object that has multiple functions/methods on it.   Then we can create an instance of that object, or struct, with a state local to just that instance.  The TogglePin example below is initialized with the hardware pin we are going to toggle. All references to functions on that object have access to that pin.  This means that toggle_pin_callback() has access to the _periodic_target of its instance. 

class TogglePin:
    """A sample target class that gets invoked as a timer callback"""

    def __init__(self, pin):
        self._periodic_target = pin

    def toggle_pin_callback(self, t):
        """the callback method"""
        self._periodic_target.value(not self._periodic_target.value())

The pin is bound to a specific TogglePin instance.  We can create as many TogglePin instances as we wish all bound to different I/O pins. Each instance has its own private state.

Passing a Function on an Object for Callbacks

    a_periodic_handler = TogglePin(Pin(2, Pin.OUT))
    a_periodic_operator = PeriodicOperator(
        Timer(-1), 500, a_periodic_handler.toggle_pin_callback

Longer Running Operations

Check the docs on how to schedule work rather than doing long running task inside the callback.

Timer Callbacks and Interrupts and the REPL

Interrupt handlers continue to run after you break into the REPL.  You can easily see this if you have a Timer callback running when you open the REPL.  The main program stops running but the callbacks continue to be invoked until the timer is terminated with deinit()

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GitHub Repositories

The code samples all came from the version of this repository as it existed at the time of writing this document

Revision History

Created 2022 12


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