python-gpiozero/gpiozero/output_devices.py

from __future__ import (
    unicode_literals,
    print_function,
    absolute_import,
    division,
)

from threading import Lock
from itertools import repeat, cycle, chain

from .exc import OutputDeviceBadValue, GPIOPinMissing
from .devices import GPIODevice, Device, CompositeDevice
from .mixins import SourceMixin
from .threads import GPIOThread


class OutputDevice(SourceMixin, GPIODevice):
    """
    Represents a generic GPIO output device.

    This class extends :class:`GPIODevice` to add facilities common to GPIO
    output devices: an :meth:`on` method to switch the device on, a
    corresponding :meth:`off` method, and a :meth:`toggle` method.

    :param int pin:
        The GPIO pin (in BCM numbering) that the device is connected to. If
        this is ``None`` a :exc:`GPIOPinMissing` will be raised.

    :param bool active_high:
        If ``True`` (the default), the :meth:`on` method will set the GPIO to
        HIGH. If ``False``, the :meth:`on` method will set the GPIO to LOW (the
        :meth:`off` method always does the opposite).

    :param bool initial_value:
        If ``False`` (the default), the device will be off initially.  If
        ``None``, the device will be left in whatever state the pin is found in
        when configured for output (warning: this can be on).  If ``True``, the
        device will be switched on initially.
    """
    def __init__(self, pin=None, active_high=True, initial_value=False):
        super(OutputDevice, self).__init__(pin)
        self._lock = Lock()
        self.active_high = active_high
        if initial_value is None:
            self.pin.function = 'output'
        else:
            self.pin.output_with_state(self._value_to_state(initial_value))

    def _value_to_state(self, value):
        return bool(self._active_state if value else self._inactive_state)

    def _write(self, value):
        try:
            self.pin.state = self._value_to_state(value)
        except AttributeError:
            self._check_open()
            raise

    def on(self):
        """
        Turns the device on.
        """
        self._write(True)

    def off(self):
        """
        Turns the device off.
        """
        self._write(False)

    def toggle(self):
        """
        Reverse the state of the device. If it's on, turn it off; if it's off,
        turn it on.
        """
        with self._lock:
            if self.is_active:
                self.off()
            else:
                self.on()

    @property
    def value(self):
        """
        Returns ``True`` if the device is currently active and ``False``
        otherwise. Setting this property changes the state of the device.
        """
        return super(OutputDevice, self).value

    @value.setter
    def value(self, value):
        self._write(value)

    @property
    def active_high(self):
        """
        When ``True``, the :attr:`value` property is ``True`` when the device's
        :attr:`pin` is high. When ``False`` the :attr:`value` property is
        ``True`` when the device's pin is low (i.e. the value is inverted).

        This property can be set after construction; be warned that changing it
        will invert :attr:`value` (i.e. changing this property doesn't change
        the device's pin state - it just changes how that state is
        interpreted).
        """
        return self._active_state

    @active_high.setter
    def active_high(self, value):
        self._active_state = True if value else False
        self._inactive_state = False if value else True

    def __repr__(self):
        try:
            return '<gpiozero.%s object on pin %r, active_high=%s, is_active=%s>' % (
                self.__class__.__name__, self.pin, self.active_high, self.is_active)
        except:
            return super(OutputDevice, self).__repr__()


class DigitalOutputDevice(OutputDevice):
    """
    Represents a generic output device with typical on/off behaviour.

    This class extends :class:`OutputDevice` with a :meth:`blink` method which
    uses an optional background thread to handle toggling the device state
    without further interaction.
    """
    def __init__(self, pin=None, active_high=True, initial_value=False):
        self._blink_thread = None
        self._controller = None
        super(DigitalOutputDevice, self).__init__(pin, active_high, initial_value)

    @property
    def value(self):
        return self._read()

    @value.setter
    def value(self, value):
        self._stop_blink()
        self._write(value)

    def close(self):
        self._stop_blink()
        super(DigitalOutputDevice, self).close()

    def on(self):
        self._stop_blink()
        self._write(True)

    def off(self):
        self._stop_blink()
        self._write(False)

    def blink(self, on_time=1, off_time=1, n=None, background=True):
        """
        Make the device turn on and off repeatedly.

        :param float on_time:
            Number of seconds on. Defaults to 1 second.

        :param float off_time:
            Number of seconds off. Defaults to 1 second.

        :param int n:
            Number of times to blink; ``None`` (the default) means forever.

        :param bool background:
            If ``True`` (the default), start a background thread to continue
            blinking and return immediately. If ``False``, only return when the
            blink is finished (warning: the default value of *n* will result in
            this method never returning).
        """
        self._stop_blink()
        self._blink_thread = GPIOThread(
            target=self._blink_device, args=(on_time, off_time, n)
        )
        self._blink_thread.start()
        if not background:
            self._blink_thread.join()
            self._blink_thread = None

    def _stop_blink(self):
        if self._controller:
            self._controller._stop_blink(self)
            self._controller = None
        if self._blink_thread:
            self._blink_thread.stop()
            self._blink_thread = None

    def _blink_device(self, on_time, off_time, n):
        iterable = repeat(0) if n is None else repeat(0, n)
        for _ in iterable:
            self._write(True)
            if self._blink_thread.stopping.wait(on_time):
                break
            self._write(False)
            if self._blink_thread.stopping.wait(off_time):
                break


class LED(DigitalOutputDevice):
    """
    Extends :class:`DigitalOutputDevice` and represents a light emitting diode
    (LED).

    Connect the cathode (short leg, flat side) of the LED to a ground pin;
    connect the anode (longer leg) to a limiting resistor; connect the other
    side of the limiting resistor to a GPIO pin (the limiting resistor can be
    placed either side of the LED).

    The following example will light the LED::

        from gpiozero import LED

        led = LED(17)
        led.on()

    :param int pin:
        The GPIO pin which the LED is attached to. See :ref:`pin-numbering` for
        valid pin numbers.

    :param bool active_high:
        If ``True`` (the default), the LED will operate normally with the
        circuit described above. If ``False`` you should wire the cathode to
        the GPIO pin, and the anode to a 3V3 pin (via a limiting resistor).

    :param bool initial_value:
        If ``False`` (the default), the LED will be off initially.  If
        ``None``, the LED will be left in whatever state the pin is found in
        when configured for output (warning: this can be on).  If ``True``, the
        LED will be switched on initially.
    """
    pass

LED.is_lit = LED.is_active


class Buzzer(DigitalOutputDevice):
    """
    Extends :class:`DigitalOutputDevice` and represents a digital buzzer
    component.

    Connect the cathode (negative pin) of the buzzer to a ground pin; connect
    the other side to any GPIO pin.

    The following example will sound the buzzer::

        from gpiozero import Buzzer

        bz = Buzzer(3)
        bz.on()

    :param int pin:
        The GPIO pin which the buzzer is attached to. See :ref:`pin-numbering`
        for valid pin numbers.

    :param bool active_high:
        If ``True`` (the default), the buzzer will operate normally with the
        circuit described above. If ``False`` you should wire the cathode to
        the GPIO pin, and the anode to a 3V3 pin.

    :param bool initial_value:
        If ``False`` (the default), the buzzer will be silent initially.  If
        ``None``, the buzzer will be left in whatever state the pin is found in
        when configured for output (warning: this can be on).  If ``True``, the
        buzzer will be switched on initially.
    """
    pass

Buzzer.beep = Buzzer.blink


class PWMOutputDevice(OutputDevice):
    """
    Generic output device configured for pulse-width modulation (PWM).

    :param int pin:
        The GPIO pin which the device is attached to. See :ref:`pin-numbering`
        for valid pin numbers.

    :param bool active_high:
        If ``True`` (the default), the :meth:`on` method will set the GPIO to
        HIGH. If ``False``, the :meth:`on` method will set the GPIO to LOW (the
        :meth:`off` method always does the opposite).

    :param float initial_value:
        If ``0`` (the default), the device's duty cycle will be 0 initially.
        Other values between 0 and 1 can be specified as an initial duty cycle.
        Note that ``None`` cannot be specified (unlike the parent class) as
        there is no way to tell PWM not to alter the state of the pin.

    :param int frequency:
        The frequency (in Hz) of pulses emitted to drive the device. Defaults
        to 100Hz.
    """
    def __init__(self, pin=None, active_high=True, initial_value=0, frequency=100):
        self._blink_thread = None
        self._controller = None
        if not 0 <= initial_value <= 1:
            raise OutputDeviceBadValue("initial_value must be between 0 and 1")
        super(PWMOutputDevice, self).__init__(pin, active_high, initial_value=None)
        try:
            # XXX need a way of setting these together
            self.pin.frequency = frequency
            self.value = initial_value
        except:
            self.close()
            raise

    def close(self):
        self._stop_blink()
        try:
            self.pin.frequency = None
        except AttributeError:
            # If the pin's already None, ignore the exception
            pass
        super(PWMOutputDevice, self).close()

    def _state_to_value(self, state):
        return float(state if self.active_high else 1 - state)

    def _value_to_state(self, value):
        return float(value if self.active_high else 1 - value)

    def _write(self, value):
        if not 0 <= value <= 1:
            raise OutputDeviceBadValue("PWM value must be between 0 and 1")
        super(PWMOutputDevice, self)._write(value)

    @property
    def value(self):
        """
        The duty cycle of the PWM device. 0.0 is off, 1.0 is fully on. Values
        in between may be specified for varying levels of power in the device.
        """
        return self._read()

    @value.setter
    def value(self, value):
        self._stop_blink()
        self._write(value)

    def on(self):
        self._stop_blink()
        self._write(1)

    def off(self):
        self._stop_blink()
        self._write(0)

    def toggle(self):
        """
        Toggle the state of the device. If the device is currently off
        (:attr:`value` is 0.0), this changes it to "fully" on (:attr:`value` is
        1.0).  If the device has a duty cycle (:attr:`value`) of 0.1, this will
        toggle it to 0.9, and so on.
        """
        self._stop_blink()
        self.value = 1 - self.value

    @property
    def is_active(self):
        """
        Returns ``True`` if the device is currently active (:attr:`value` is
        non-zero) and ``False`` otherwise.
        """
        return self.value != 0

    @property
    def frequency(self):
        """
        The frequency of the pulses used with the PWM device, in Hz. The
        default is 100Hz.
        """
        return self.pin.frequency

    @frequency.setter
    def frequency(self, value):
        self.pin.frequency = value

    def blink(
            self, on_time=1, off_time=1, fade_in_time=0, fade_out_time=0,
            n=None, background=True):
        """
        Make the device turn on and off repeatedly.

        :param float on_time:
            Number of seconds on. Defaults to 1 second.

        :param float off_time:
            Number of seconds off. Defaults to 1 second.

        :param float fade_in_time:
            Number of seconds to spend fading in. Defaults to 0.

        :param float fade_out_time:
            Number of seconds to spend fading out. Defaults to 0.

        :param int n:
            Number of times to blink; ``None`` (the default) means forever.

        :param bool background:
            If ``True`` (the default), start a background thread to continue
            blinking and return immediately. If ``False``, only return when the
            blink is finished (warning: the default value of *n* will result in
            this method never returning).
        """
        self._stop_blink()
        self._blink_thread = GPIOThread(
            target=self._blink_device,
            args=(on_time, off_time, fade_in_time, fade_out_time, n)
        )
        self._blink_thread.start()
        if not background:
            self._blink_thread.join()
            self._blink_thread = None

    def pulse(self, fade_in_time=1, fade_out_time=1, n=None, background=True):
        """
        Make the device fade in and out repeatedly.

        :param float fade_in_time:
            Number of seconds to spend fading in. Defaults to 1.

        :param float fade_out_time:
            Number of seconds to spend fading out. Defaults to 1.

        :param int n:
            Number of times to pulse; ``None`` (the default) means forever.

        :param bool background:
            If ``True`` (the default), start a background thread to continue
            pulsing and return immediately. If ``False``, only return when the
            pulse is finished (warning: the default value of *n* will result in
            this method never returning).
        """
        on_time = off_time = 0
        self.blink(
            on_time, off_time, fade_in_time, fade_out_time, n, background
        )

    def _stop_blink(self):
        if self._controller:
            self._controller._stop_blink(self)
            self._controller = None
        if self._blink_thread:
            self._blink_thread.stop()
            self._blink_thread = None

    def _blink_device(
            self, on_time, off_time, fade_in_time, fade_out_time, n, fps=25):
        sequence = []
        if fade_in_time > 0:
            sequence += [
                (i * (1 / fps) / fade_in_time, 1 / fps)
                for i in range(int(fps * fade_in_time))
                ]
        sequence.append((1, on_time))
        if fade_out_time > 0:
            sequence += [
                (1 - (i * (1 / fps) / fade_out_time), 1 / fps)
                for i in range(int(fps * fade_out_time))
                ]
        sequence.append((0, off_time))
        sequence = (
                cycle(sequence) if n is None else
                chain.from_iterable(repeat(sequence, n))
                )
        for value, delay in sequence:
            self._write(value)
            if self._blink_thread.stopping.wait(delay):
                break


class PWMLED(PWMOutputDevice):
    """
    Extends :class:`PWMOutputDevice` and represents a light emitting diode
    (LED) with variable brightness.

    A typical configuration of such a device is to connect a GPIO pin to the
    anode (long leg) of the LED, and the cathode (short leg) to ground, with
    an optional resistor to prevent the LED from burning out.

    :param int pin:
        The GPIO pin which the LED is attached to. See :ref:`pin-numbering` for
        valid pin numbers.

    :param bool active_high:
        If ``True`` (the default), the :meth:`on` method will set the GPIO to
        HIGH. If ``False``, the :meth:`on` method will set the GPIO to LOW (the
        :meth:`off` method always does the opposite).

    :param float initial_value:
        If ``0`` (the default), the LED will be off initially. Other values
        between 0 and 1 can be specified as an initial brightness for the LED.
        Note that ``None`` cannot be specified (unlike the parent class) as
        there is no way to tell PWM not to alter the state of the pin.

    :param int frequency:
        The frequency (in Hz) of pulses emitted to drive the LED. Defaults
        to 100Hz.
    """
    pass

PWMLED.is_lit = PWMLED.is_active


def _led_property(index, doc=None):
    def getter(self):
        return self._leds[index].value
    def setter(self, value):
        self._stop_blink()
        self._leds[index].value = value
    return property(getter, setter, doc=doc)


class RGBLED(SourceMixin, Device):
    """
    Extends :class:`Device` and represents a full color LED component (composed
    of red, green, and blue LEDs).

    Connect the common cathode (longest leg) to a ground pin; connect each of
    the other legs (representing the red, green, and blue anodes) to any GPIO
    pins.  You can either use three limiting resistors (one per anode) or a
    single limiting resistor on the cathode.

    The following code will make the LED purple::

        from gpiozero import RGBLED

        led = RGBLED(2, 3, 4)
        led.color = (1, 0, 1)

    :param int red:
        The GPIO pin that controls the red component of the RGB LED.

    :param int green:
        The GPIO pin that controls the green component of the RGB LED.

    :param int blue:
        The GPIO pin that controls the blue component of the RGB LED.

    :param bool active_high:
        Set to ``True`` (the default) for common cathode RGB LEDs. If you are
        using a common anode RGB LED, set this to ``False``.

    :param tuple initial_value:
        The initial color for the RGB LED. Defaults to black ``(0, 0, 0)``.

    :param bool pwm:
        If ``True`` (the default), construct :class:`PWMLED` instances for
        each component of the RGBLED. If ``False``, construct regular
        :class:`LED` instances, which prevents smooth color graduations.
    """
    def __init__(
            self, red=None, green=None, blue=None, active_high=True,
            initial_value=(0, 0, 0), pwm=True):
        self._leds = ()
        self._blink_thread = None
        if not all(p is not None for p in [red, green, blue]):
            raise GPIOPinMissing('red, green, and blue pins must be provided')
        LEDClass = PWMLED if pwm else LED
        super(RGBLED, self).__init__()
        self._leds = tuple(LEDClass(pin, active_high) for pin in (red, green, blue))
        self.value = initial_value

    red = _led_property(0)
    green = _led_property(1)
    blue = _led_property(2)

    def close(self):
        if self._leds:
            self._stop_blink()
            for led in self._leds:
                led.close()
            self._leds = ()
        super(RGBLED, self).close()

    @property
    def closed(self):
        return len(self._leds) == 0

    @property
    def value(self):
        """
        Represents the color of the LED as an RGB 3-tuple of ``(red, green,
        blue)`` where each value is between 0 and 1 if ``pwm`` was ``True``
        when the class was constructed (and only 0 or 1 if not).

        For example, purple would be ``(1, 0, 1)`` and yellow would be ``(1, 1,
        0)``, while orange would be ``(1, 0.5, 0)``.
        """
        return (self.red, self.green, self.blue)

    @value.setter
    def value(self, value):
        for component in value:
            if not 0 <= component <= 1:
                raise OutputDeviceBadValue('each RGB color component must be between 0 and 1')
            if isinstance(self._leds[0], LED):
                if component not in (0, 1):
                    raise OutputDeviceBadValue('each RGB color component must be 0 or 1 with non-PWM RGBLEDs')
        self._stop_blink()
        self.red, self.green, self.blue = value

    @property
    def is_active(self):
        """
        Returns ``True`` if the LED is currently active (not black) and
        ``False`` otherwise.
        """
        return self.value != (0, 0, 0)

    is_lit = is_active
    color = value

    def on(self):
        """
        Turn the LED on. This equivalent to setting the LED color to white
        ``(1, 1, 1)``.
        """
        self.value = (1, 1, 1)

    def off(self):
        """
        Turn the LED off. This is equivalent to setting the LED color to black
        ``(0, 0, 0)``.
        """
        self.value = (0, 0, 0)

    def toggle(self):
        """
        Toggle the state of the device. If the device is currently off
        (:attr:`value` is ``(0, 0, 0)``), this changes it to "fully" on
        (:attr:`value` is ``(1, 1, 1)``).  If the device has a specific color,
        this method inverts the color.
        """
        r, g, b = self.value
        self.value = (1 - r, 1 - g, 1 - b)

    def blink(
            self, on_time=1, off_time=1, fade_in_time=0, fade_out_time=0,
            on_color=(1, 1, 1), off_color=(0, 0, 0), n=None, background=True):
        """
        Make the device turn on and off repeatedly.

        :param float on_time:
            Number of seconds on. Defaults to 1 second.

        :param float off_time:
            Number of seconds off. Defaults to 1 second.

        :param float fade_in_time:
            Number of seconds to spend fading in. Defaults to 0. Must be 0 if
            ``pwm`` was ``False`` when the class was constructed
            (:exc:`ValueError` will be raised if not).

        :param float fade_out_time:
            Number of seconds to spend fading out. Defaults to 0. Must be 0 if
            ``pwm`` was ``False`` when the class was constructed
            (:exc:`ValueError` will be raised if not).

        :param tuple on_color:
            The color to use when the LED is "on". Defaults to white.

        :param tuple off_color:
            The color to use when the LED is "off". Defaults to black.

        :param int n:
            Number of times to blink; ``None`` (the default) means forever.

        :param bool background:
            If ``True`` (the default), start a background thread to continue
            blinking and return immediately. If ``False``, only return when the
            blink is finished (warning: the default value of *n* will result in
            this method never returning).
        """
        if isinstance(self._leds[0], LED):
            if fade_in_time:
                raise ValueError('fade_in_time must be 0 with non-PWM RGBLEDs')
            if fade_out_time:
                raise ValueError('fade_out_time must be 0 with non-PWM RGBLEDs')
        self._stop_blink()
        self._blink_thread = GPIOThread(
            target=self._blink_device,
            args=(
                on_time, off_time, fade_in_time, fade_out_time,
                on_color, off_color, n
            )
        )
        self._blink_thread.start()
        if not background:
            self._blink_thread.join()
            self._blink_thread = None

    def pulse(
            self, fade_in_time=1, fade_out_time=1,
            on_color=(1, 1, 1), off_color=(0, 0, 0), n=None, background=True):
        """
        Make the device fade in and out repeatedly.

        :param float fade_in_time:
            Number of seconds to spend fading in. Defaults to 1.

        :param float fade_out_time:
            Number of seconds to spend fading out. Defaults to 1.

        :param tuple on_color:
            The color to use when the LED is "on". Defaults to white.

        :param tuple off_color:
            The color to use when the LED is "off". Defaults to black.

        :param int n:
            Number of times to pulse; ``None`` (the default) means forever.

        :param bool background:
            If ``True`` (the default), start a background thread to continue
            pulsing and return immediately. If ``False``, only return when the
            pulse is finished (warning: the default value of *n* will result in
            this method never returning).
        """
        on_time = off_time = 0
        self.blink(
            on_time, off_time, fade_in_time, fade_out_time,
            on_color, off_color, n, background
        )

    def _stop_blink(self, led=None):
        # If this is called with a single led, we stop all blinking anyway
        if self._blink_thread:
            self._blink_thread.stop()
            self._blink_thread = None

    def _blink_device(
            self, on_time, off_time, fade_in_time, fade_out_time, on_color,
            off_color, n, fps=25):
        # Define some simple lambdas to perform linear interpolation between
        # off_color and on_color
        lerp = lambda t, fade_in: tuple(
            (1 - t) * off + t * on
            if fade_in else
            (1 - t) * on + t * off
            for off, on in zip(off_color, on_color)
            )
        sequence = []
        if fade_in_time > 0:
            sequence += [
                (lerp(i * (1 / fps) / fade_in_time, True), 1 / fps)
                for i in range(int(fps * fade_in_time))
                ]
        sequence.append((on_color, on_time))
        if fade_out_time > 0:
            sequence += [
                (lerp(i * (1 / fps) / fade_out_time, False), 1 / fps)
                for i in range(int(fps * fade_out_time))
                ]
        sequence.append((off_color, off_time))
        sequence = (
                cycle(sequence) if n is None else
                chain.from_iterable(repeat(sequence, n))
                )
        for l in self._leds:
            l._controller = self
        for value, delay in sequence:
            for l, v in zip(self._leds, value):
                l._write(v)
            if self._blink_thread.stopping.wait(delay):
                break


class Motor(SourceMixin, CompositeDevice):
    """
    Extends :class:`CompositeDevice` and represents a generic motor
    connected to a bi-directional motor driver circuit (i.e.  an `H-bridge`_).

    Attach an `H-bridge`_ motor controller to your Pi; connect a power source
    (e.g. a battery pack or the 5V pin) to the controller; connect the outputs
    of the controller board to the two terminals of the motor; connect the
    inputs of the controller board to two GPIO pins.

    .. _H-bridge: https://en.wikipedia.org/wiki/H_bridge

    The following code will make the motor turn "forwards"::

        from gpiozero import Motor

        motor = Motor(17, 18)
        motor.forward()

    :param int forward:
        The GPIO pin that the forward input of the motor driver chip is
        connected to.

    :param int backward:
        The GPIO pin that the backward input of the motor driver chip is
        connected to.

    :param bool pwm:
        If ``True`` (the default), construct :class:`PWMOutputDevice`
        instances for the motor controller pins, allowing both direction and
        variable speed control. If ``False``, construct
        :class:`DigitalOutputDevice` instances, allowing only direction
        control.
    """
    def __init__(self, forward=None, backward=None, pwm=True):
        if not all(p is not None for p in [forward, backward]):
            raise GPIOPinMissing(
                'forward and backward pins must be provided'
            )
        PinClass = PWMOutputDevice if pwm else DigitalOutputDevice
        super(Motor, self).__init__(
                forward_device=PinClass(forward),
                backward_device=PinClass(backward),
                _order=('forward_device', 'backward_device'))

    @property
    def value(self):
        """
        Represents the speed of the motor as a floating point value between -1
        (full speed backward) and 1 (full speed forward), with 0 representing
        stopped.
        """
        return self.forward_device.value - self.backward_device.value

    @value.setter
    def value(self, value):
        if not -1 <= value <= 1:
            raise OutputDeviceBadValue("Motor value must be between -1 and 1")
        if value > 0:
            try:
                self.forward(value)
            except ValueError as e:
                raise OutputDeviceBadValue(e)
        elif value < 0:
            try:
               self.backward(-value)
            except ValueError as e:
                raise OutputDeviceBadValue(e)
        else:
            self.stop()

    @property
    def is_active(self):
        """
        Returns ``True`` if the motor is currently running and ``False``
        otherwise.
        """
        return self.value != 0

    def forward(self, speed=1):
        """
        Drive the motor forwards.

        :param float speed:
            The speed at which the motor should turn. Can be any value between
            0 (stopped) and the default 1 (maximum speed) if ``pwm`` was
            ``True`` when the class was constructed (and only 0 or 1 if not).
        """
        if not 0 <= speed <= 1:
            raise ValueError('forward speed must be between 0 and 1')
        if isinstance(self.forward_device, DigitalOutputDevice):
            if speed not in (0, 1):
                raise ValueError('forward speed must be 0 or 1 with non-PWM Motors')
        self.backward_device.off()
        self.forward_device.value = speed

    def backward(self, speed=1):
        """
        Drive the motor backwards.

        :param float speed:
            The speed at which the motor should turn. Can be any value between
            0 (stopped) and the default 1 (maximum speed) if ``pwm`` was
            ``True`` when the class was constructed (and only 0 or 1 if not).
        """
        if not 0 <= speed <= 1:
            raise ValueError('backward speed must be between 0 and 1')
        if isinstance(self.backward_device, DigitalOutputDevice):
            if speed not in (0, 1):
                raise ValueError('backward speed must be 0 or 1 with non-PWM Motors')
        self.forward_device.off()
        self.backward_device.value = speed

    def reverse(self):
        """
        Reverse the current direction of the motor. If the motor is currently
        idle this does nothing. Otherwise, the motor's direction will be
        reversed at the current speed.
        """
        self.value = -self.value

    def stop(self):
        """
        Stop the motor.
        """
        self.forward_device.off()
        self.backward_device.off()


class Servo(SourceMixin, CompositeDevice):
    """
    Extends :class:`CompositeDevice` and represents a PWM-controlled servo
    motor connected to a GPIO pin.

    Connect a power source (e.g. a battery pack or the 5V pin) to the power
    cable of the servo (this is typically colored red); connect the ground
    cable of the servo (typically colored black or brown) to the negative of
    your battery pack, or a GND pin; connect the final cable (typically colored
    white or orange) to the GPIO pin you wish to use for controlling the servo.

    The following code will make the servo move between its minimum, maximum,
    and mid-point positions with a pause between each::

        from gpiozero import Servo
        from time import sleep

        servo = Servo(17)
        while True:
            servo.min()
            sleep(1)
            servo.mid()
            sleep(1)
            servo.max()
            sleep(1)

    :param int pin:
        The GPIO pin which the device is attached to. See :ref:`pin-numbering`
        for valid pin numbers.

    :param float initial_value:
        If ``0`` (the default), the device's mid-point will be set
        initially.  Other values between -1 and +1 can be specified as an
        initial position. ``None`` means to start the servo un-controlled (see
        :attr:`value`).

    :param float min_pulse_width:
        The pulse width corresponding to the servo's minimum position. This
        defaults to 1ms.

    :param float max_pulse_width:
        The pulse width corresponding to the servo's maximum position. This
        defaults to 2ms.

    :param float frame_width:
        The length of time between servo control pulses measured in seconds.
        This defaults to 20ms which is a common value for servos.
    """
    def __init__(
            self, pin=None, initial_value=0.0,
            min_pulse_width=1/1000, max_pulse_width=2/1000,
            frame_width=20/1000):
        if min_pulse_width >= max_pulse_width:
            raise ValueError('min_pulse_width must be less than max_pulse_width')
        if max_pulse_width >= frame_width:
            raise ValueError('max_pulse_width must be less than frame_width')
        self._frame_width = frame_width
        self._min_dc = min_pulse_width / frame_width
        self._dc_range = (max_pulse_width - min_pulse_width) / frame_width
        self._min_value = -1
        self._value_range = 2
        super(Servo, self).__init__(
            pwm_device=PWMOutputDevice(pin, frequency=int(1 / frame_width)))
        try:
            self.value = initial_value
        except:
            self.close()
            raise

    @property
    def frame_width(self):
        """
        The time between control pulses, measured in seconds.
        """
        return self._frame_width

    @property
    def min_pulse_width(self):
        """
        The control pulse width corresponding to the servo's minimum position,
        measured in seconds.
        """
        return self._min_dc * self.frame_width

    @property
    def max_pulse_width(self):
        """
        The control pulse width corresponding to the servo's maximum position,
        measured in seconds.
        """
        return (self._dc_range * self.frame_width) + self.min_pulse_width

    @property
    def pulse_width(self):
        """
        Returns the current pulse width controlling the servo.
        """
        if self.pwm_device.pin.frequency is None:
            return None
        else:
            return self.pwm_device.pin.state * self.frame_width

    def min(self):
        """
        Set the servo to its minimum position.
        """
        self.value = -1

    def mid(self):
        """
        Set the servo to its mid-point position.
        """
        self.value = 0

    def max(self):
        """
        Set the servo to its maximum position.
        """
        self.value = 1

    def detach(self):
        """
        Temporarily disable control of the servo. This is equivalent to
        setting :attr:`value` to ``None``.
        """
        self.value = None

    def _get_value(self):
        if self.pwm_device.pin.frequency is None:
            return None
        else:
            return (
                ((self.pwm_device.pin.state - self._min_dc) / self._dc_range) *
                self._value_range + self._min_value)

    @property
    def value(self):
        """
        Represents the position of the servo as a value between -1 (the minimum
        position) and +1 (the maximum position). This can also be the special
        value ``None`` indicating that the servo is currently "uncontrolled",
        i.e. that no control signal is being sent. Typically this means the
        servo's position remains unchanged, but that it can be moved by hand.
        """
        result = self._get_value()
        if result is None:
            return result
        else:
            # NOTE: This round() only exists to ensure we don't confuse people
            # by returning 2.220446049250313e-16 as the default initial value
            # instead of 0. The reason _get_value and _set_value are split
            # out is for descendents that require the un-rounded values for
            # accuracy
            return round(result, 14)

    @value.setter
    def value(self, value):
        if value is None:
            self.pwm_device.pin.frequency = None
        elif -1 <= value <= 1:
            self.pwm_device.pin.frequency = int(1 / self.frame_width)
            self.pwm_device.pin.state = (
                self._min_dc + self._dc_range *
                ((value - self._min_value) / self._value_range)
                )
        else:
            raise OutputDeviceBadValue(
                "Servo value must be between -1 and 1, or None")

    @property
    def is_active(self):
        return self.value is not None


class AngularServo(Servo):
    """
    Extends :class:`Servo` and represents a rotational PWM-controlled servo
    motor which can be set to particular angles (assuming valid minimum and
    maximum angles are provided to the constructor).

    Connect a power source (e.g. a battery pack or the 5V pin) to the power
    cable of the servo (this is typically colored red); connect the ground
    cable of the servo (typically colored black or brown) to the negative of
    your battery pack, or a GND pin; connect the final cable (typically colored
    white or orange) to the GPIO pin you wish to use for controlling the servo.

    Next, calibrate the angles that the servo can rotate to. In an interactive
    Python session, construct a :class:`Servo` instance. The servo should move
    to its mid-point by default. Set the servo to its minimum value, and
    measure the angle from the mid-point. Set the servo to its maximum value,
    and again measure the angle::

        >>> from gpiozero import Servo
        >>> s = Servo(17)
        >>> s.min() # measure the angle
        >>> s.max() # measure the angle

    You should now be able to construct an :class:`AngularServo` instance
    with the correct bounds::

        >>> from gpiozero import AngularServo
        >>> s = AngularServo(17, min_angle=-42, max_angle=44)
        >>> s.angle = 0.0
        >>> s.angle
        0.0
        >>> s.angle = 15
        >>> s.angle
        15.0

    .. note::

        You can set *min_angle* greater than *max_angle* if you wish to reverse
        the sense of the angles (e.g. ``min_angle=45, max_angle=-45``). This
        can be useful with servos that rotate in the opposite direction to your
        expectations of minimum and maximum.

    :param int pin:
        The GPIO pin which the device is attached to. See :ref:`pin-numbering`
        for valid pin numbers.

    :param float initial_angle:
        Sets the servo's initial angle to the specified value. The default is
        0. The value specified must be between *min_angle* and *max_angle*
        inclusive. ``None`` means to start the servo un-controlled (see
        :attr:`value`).

    :param float min_angle:
        Sets the minimum angle that the servo can rotate to. This defaults to
        -90, but should be set to whatever you measure from your servo during
        calibration.

    :param float max_angle:
        Sets the maximum angle that the servo can rotate to. This defaults to
        90, but should be set to whatever you measure from your servo during
        calibration.

    :param float min_pulse_width:
        The pulse width corresponding to the servo's minimum position. This
        defaults to 1ms.

    :param float max_pulse_width:
        The pulse width corresponding to the servo's maximum position. This
        defaults to 2ms.

    :param float frame_width:
        The length of time between servo control pulses measured in seconds.
        This defaults to 20ms which is a common value for servos.
    """
    def __init__(
            self, pin=None, initial_angle=0.0,
            min_angle=-90, max_angle=90,
            min_pulse_width=1/1000, max_pulse_width=2/1000,
            frame_width=20/1000):
        self._min_angle = min_angle
        self._angular_range = max_angle - min_angle
        initial_value = 2 * ((initial_angle - min_angle) / self._angular_range) - 1
        super(AngularServo, self).__init__(
            pin, initial_value, min_pulse_width, max_pulse_width, frame_width)

    @property
    def min_angle(self):
        """
        The minimum angle that the servo will rotate to when :meth:`min` is
        called.
        """
        return self._min_angle

    @property
    def max_angle(self):
        """
        The maximum angle that the servo will rotate to when :meth:`max` is
        called.
        """
        return self._min_angle + self._angular_range

    @property
    def angle(self):
        """
        The position of the servo as an angle measured in degrees. This will
        only be accurate if *min_angle* and *max_angle* have been set
        appropriately in the constructor.

        This can also be the special value ``None`` indicating that the servo
        is currently "uncontrolled", i.e. that no control signal is being sent.
        Typically this means the servo's position remains unchanged, but that
        it can be moved by hand.
        """
        result = self._get_value()
        if result is None:
            return None
        else:
            # NOTE: Why round(n, 12) here instead of 14? Angle ranges can be
            # much larger than -1..1 so we need a little more rounding to
            # smooth off the rough corners!
            return round(
                self._angular_range *
                ((result - self._min_value) / self._value_range) +
                self._min_angle, 12)

    @angle.setter
    def angle(self, value):
        if value is None:
            self.value = None
        else:
            self.value = (
                self._value_range *
                ((value - self._min_angle) / self._angular_range) +
                self._min_value)