EuroPi - Andy's Utility Classes


My Trigger to Gate script for EuroPi uses some utility functions I developed, that might be useful for other EuroPi scripts:

  • KnobWithHysteresis
  • KnobWithPassThrough
  • Scheduler

Rather than each EuroPi script inventing its own hyseresis mitigation and knob pass-through logic etc., these utility classes can be used by any EuroPi script that needs them.

You can install them into the lib/contrib/experimental directory of the EuroPi software, or in a separate lib/contrib/utils directory. Indeed, you can put them wherever you want - even copy and paste the classes into the body of the EuroPi script you are developing.

The problems these utility classes solve

Hysteresis Mitigation

The term hysteresis refers to the problem of the knob value flickering when the knob is not being turned. This is a common problem with rotary encoders and can be mitigated by only accepting new values when the knob has been turned by a certain amount. This is especially useful when you have a knob that is used to set a value that is displayed on a screen, as in the Trigger to Gate script.

The knobs in my Trigger to Gate script use hysteresis mitigation which prevents the values flickering, even though you are not turning the knob. There is a 1 second timeout when you can dial in the exact value you want, then the knob value will “lock”. To unlock the knob value, turn the knob past the threshold again.

Pass-through knob values

The term “pass-through” refers to the technique of enabling a parameter’s value only when the knob physical position passes through the existing value, which prevents sudden jumps in values when you switch screen modes.

If knobs are used in two different modes, as they are in my Trigger to Gate script, we run into the problem of the physical knob position not matching the last value of that mode, when you switch between modes (between trigger to gate mode and clock mode, for example). To solve this, the knobs use “pass-through” logic, which you may be familiar with when loading presets into a hardware synthesiser.

If turning a knob doesn’t change the value, it’s because the knob is not yet “passed-through” the current value. Simply turn the knob until it does change. For example, if the existing value is 0 and your physical knob position is at “5 o’clock” then you need to turn the physical knob counter-clockwise until it passes through 0 and then clockwise again to your required value.


When writing a EuroPi script, you may need to run tasks at a given time in the future. For example, you may want to turn off a gate after a certain time, or run a task every second. Most EuroPi scripts will have a simple while loop that runs continuously, with various ad-hoc flags and variables managing the behaviour inside the loop. This can get messy and hard to understand.

My scheduler class offers a more organised way of looping and scheduling tasks to run at a given time in the future. A task is a function that you want to run at a given time in the future, thus instead of having a while loop and global variables, you have a function per task and a time to run it.

Another potentially interesting way of managing tasks is to use a game inspired Entity Component System loop, as I describe in my article TodoMVC implemented using a game architecture — ECS. I leave that as an exercise for the future.

Slow screen updates in EuroPi

Screen updates in EuroPi are a blocking operation and take a long time relative to the millisecond level of activity of signals and interrupts. This means incoming triggers can easily be missed and interrupts won’t fire. The scheduler class can help with this problem. For example, you can schedule to update the screen at a regular interval, rather than updating the screen every time you change a value. Even better, you can use the scheduler to update the screen only when the value has changed, rather than every time through the loop. This is accomplished by scheduling the screen update task only when needed, and ensure that the screen update task function does not reschedule itself.

If you are not using the Scheduler class, you should use a flag to indicate when the screen needs updating and only update the screen when the flag is set.

NOTE: MicroPython async is supported on the Raspberry Pi Pico, and can help with task management. However even when using async, there is only one thread involved so you still need to be careful with blocking operations like writing to the EuroPi screen.

Yes, there is a second thread available on the Raspberry Pi Pico but MicroPython support for this feature is unreliable but will hopefully will improve in the future (we have been hoping for this for a long time though 2021-2024 and it is still a problem).

The utility classes presented here do not use async, nor do they use the second thread.

Here are the utility classes:

class KnobWithHysteresis

This is a class to cure the hysteresis problem with the rotary encoder knobs k1 and k2 of the EuroPi.

It is easy to use because you just wrap the knob in the class and it will automatically mitigate the hysteresis problem. The knob will only change value when you turn it by a certain amount, which you can set with the tolerance.

from europi import k1, k2

k1 = KnobWithHysteresis(k1)  # backwards compatible, no hyteresis mitigation
k1 = KnobWithHysteresis(k1, tolerance=2)  # set tolerance to 2, hysteresis mitigation

You can set any value within the lock_delay time period (an additional constructor parameter that defaults to 1000ms) and it will be accepted but once the lock_delay time period expires the value returned will be locked and you have to move the knob by at least tolerance to change the value, at which point the time period before re-locking is extended by lock_delay time (defaults to 1s).

Starts locked on the first value it sees, to avoid jittery beginning values.


You can include this class in your EuroPi script to cure the hysteresis problem with the rotary encoder. You can put the code in a separate file and import it into your script. e.g. from knob_with_hysteresis import KnobWithHysteresis where your file contains the following code:

import utime

class KnobWithHysteresis:
    This is a class to cure the hysteresis problem with the rotary encoder.
    def __init__(self, knob, tolerance=0, lock_delay=1000, name=None) -> None:
        self.knob = knob
        self.tolerance = tolerance
        self.lock_delay = lock_delay
        self.lock_time = utime.ticks_ms() # initially locked = name if name else "unnamed" # for debugging
        self.value = None  # cached value when locked, when None the first reading is used even when locked

    def locked(self):
        now = utime.ticks_ms()
        time_left = utime.ticks_diff(self.lock_time, now) # lock_time - now
        _time_expired = time_left <= 0
        return _time_expired

    def _update_value_if_allowed(self, new_value):
        if self.value is None or self._allow(self.value, new_value):
            self.value = new_value
        return self.value

    def _allow(self, old_value, new_value):
        if self.tolerance == 0:
            return True  # backwards compatibility
        if not self.locked:
            return True  # allow any value to get in

        # at this point the lock_time has expired
        big_enough_change = abs(old_value - new_value) >= self.tolerance
        if big_enough_change:
            now = utime.ticks_ms()
            self.lock_time = now + self.lock_delay
            self.locked_msg_shown_debug = False
            return True

        return False

    def choice(self, *args):
        new_value = self.knob.choice(*args)
        return self._update_value_if_allowed(new_value)

This class is a wrapper around the EuroPi Knob class (rather than inheriting), so if you need to call additional Knob methods, or the methods of its superclass AnalogueReader, you can do so by declaring those methods here and forwarding the calls to the wrapped Knob instance. Alternatively you can simply call the methods directly on the wrapped knob instance. e.g. k1_wrapped.knob.xxxxx.


Wrapping a EuroPi k1 knob with the KnobWithHysteresis class:

# Wrap knobs in KnobWithHysteresis to avoid jitter.
self.k1_gate_length = KnobWithHysteresis(k1, tolerance=2, name="k1_gate_length")
self.k2_gate_delay = KnobWithHysteresis(k2, tolerance=2, name="k2_gate_delay")

class KnobWithPassThrough

Disable changing value till knob is moved and “passes-through” the current cached value. Useful for when you have a knob that is used in two different modes and you don’t want the value to jump when you switch modes (think recalling presets on a hardware synth).

The knob passthrough feature is also useful when you turn on your hardware module and you want the knob value to be in the same position as when you turned it off, and the knob has physically moved slightly, so your sound is unfortunately, different.

The knob value will initiallly lock to the initial_value parameter, which defaults to 50. Moving the underlying knob by any amount will overwrite that initial_value. If pass-through is enabled, then you will have to move the underlying knob “through” the initial value to begin changing it.

Its actually easier to use than to explain!


Simplest usage is to wrap a knob with this class, passing the initial value e.g.

from europi import k1, k2

choices = list(range(0, 200))

k = KnobWithPassThrough(k1, initial=50)
val = k.choice(choices)  # val will remain 50 until k1 is moved

If you want k1 to control two different values depending on a mode, you need to create two instances of this class, wrapping the same knob e.g.

from europi import k1, k2

k_mode1 = KnobWithPassThrough(k1, initial=50)
k_mode2 = KnobWithPassThrough(k1, initial=100)

Pass through requirement is initially turned off, and only activated when you have “switched modes” and need the same knob to drive different values. You switch modes by calling mode_changed() on the (KnobWithPassThrough) knob instance you are switching to.

When you switch modes e.g. as a result of a button press, you need to call mode_changed() on the mode you are switching to, in order to tell it to use its cached value until physical knob pass-through has ocurred e.g.

def button_click():
    mode = 2 if mode == 1 else 1
    if mode == 1:
    elif mode == 2:

# Read from the correct (KnobWithPassThrough) knob depending on mode
# Reading from the other modes's KnobWithPassThrough knob is undefined
if mode == 1:
    val = k_mode1.choice(choices)
elif mode == 2:
    val = k_mode2.choice(choices)

You can include this class in your EuroPi script to cure the knob pass-through problem with the rotary encoder. You can put the code in a separate file and import it into your script. e.g. from knob_with_pass_through import KnobWithPassThrough where your file contains the following code:

class KnobWithPassThrough:
    Disable changing value till knob is moved and "passes-through" the current cached value.
    def __init__(self, knob, initial_value=50) -> None:
        self.knob = knob
        self.value = initial_value # cached value when locked
        self.locked = False # stay on value until the knob meets unlock condition
        self.unlock_condition = 'any change'  # '<' or '>' or 'any change'
        self.locked_on_knob_value = None # remember knob value we initially locked on when setting to 'any change'
        self.recalc_pending = False # unlock_condition needs recalculating

    def mode_changed(self):
        # call this when switching to a new mode that uses the same underlying knob
        self.recalc_pending = True

    def _recalc_pass_through_condition(self, current_knob_value):
        self.locked = True
        self.unlock_condition = '<' if current_knob_value > self.value else '>'

    def _has_passed_through(self, value):
        if self.unlock_condition == '<':
            return value <= self.value
        elif self.unlock_condition == '>':
            return value >= self.value
        elif self.unlock_condition == 'any change':
            return value != self.locked_on_knob_value

    def _update_pass_through(self, new_value):
        if self.recalc_pending:
            self.recalc_pending = False
        if self.locked_on_knob_value is None:
            self.locked_on_knob_value = new_value
            self.locked = True
            return self.value
        if self.locked and self._has_passed_through(new_value):
            self.locked = False

        if self.locked:
            # return the cached value
            return self.value
            # pass through underlying knob new value, update cache
            self.value = new_value
            return self.value

    def choice(self, *args, **kwargs):
        new_value = self.knob.choice(*args)
        return self._update_pass_through(new_value)


Wrapping a EuroPi k1 knob with the KnobWithPassThrough class:

# Wrap knobs in KnobWithPassThrough to prevent values jumping when toggling modes.
self.k1_gate_length = KnobWithPassThrough(k1, initial_value=200)
self.k2_gate_delay = KnobWithPassThrough(k2, initial_value=0)

Tip: You can wrap a knob with both KnobWithHysteresis and KnobWithPassThrough classes, if you need both features. E.g. k1 = KnobWithHysteresis(KnobWithPassThrough(k1)).

class Scheduler

A simple scheduler for running tasks at a given time in the future.

import utime

s = Scheduler()

s.schedule_task(some_function, ms=1000)  # run some_function in 1 second
s.schedule_task(some_other_function, ms=2000)  # run some_other_function in 2 seconds

while s.enabled:
    utime.sleep_ms(10) # the lower this is, the more accurate the schedule will be

def some_function():
    print("some_function called")
    s.schedule_task(some_function, ms=1000)  # reschedule some_function to run in 1 second

You can stop your loop using s.stop() which sets s.enabled to False, or just interrupt it with Ctrl-C. Its really up to you how you implement the loop.

Callbacks can be any callable object, e.g. a function, a method, a lambda, etc. No support for passing arguments to callbacks yet.

Implementation Note: MicroPython does not support methods having reference equality (regular functions are ok) so we have to compare callbacks using the callback’s function name string rather than by the callback function object itself.


You can include this class in your EuroPi script to run tasks at a given time in the future. You can put the code in a separate file and import it into your script. e.g. from scheduler import Scheduler where your file contains the following code:

import utime

class Scheduler:
    A simple scheduler for running tasks at a given time in the future.

    def __init__(self):
        self.enabled = True
        self.schedule = [] # list of tuples (time, callback, callback_func_name)

    def add_task(self, callback, ms:int=0):
        self.schedule.append((utime.ticks_add(utime.ticks_ms(), ms), callback, callback.__name__))

    def remove_task(self, callback, must_be_found=False):
        to_remove = []
        found = False
        for scheduled_time, cb, callback_func_name in self.schedule:
            if callback_func_name == callback.__name__:
                to_remove.append((scheduled_time, cb, callback_func_name))
                found = True
        for item in to_remove:
        if not found and must_be_found:
            print(f"cannot remove task {callback} viz. {callback.__name__} from schedule", len(self.schedule), "tasks in schedule")

    def run_once(self):
        now = utime.ticks_ms()
        for scheduled_time, callback, _ in self.schedule:
            if utime.ticks_diff(scheduled_time, now) <= 0:
                self.schedule.remove((scheduled_time, callback, _))

    def stop(self):
        self.enabled = False

    def print_schedule(self):
        print('  Schedule:')
        for scheduled_time, callback, callback_func_name in self.schedule:
            print(f"  {callback_func_name} {scheduled_time}")

For fuller examples of how to use all these classes together, see my Trigger to Gate script.


  • Trigger to Gate script, with gate delay feature and internal clock mode.
  • Trigger Gate Delay script, without the clock mode functionality.
  • Documentation on my Scheduler, Hysteresis Mitigation and Knob Pass Through utility classes, which you can use in your own EuroPi scripts.

Last modified June 3, 2024: europi table of variants (6dc2027)