docs/library/machine: Specify initial machine.PWM class.

This adds an initial specification of the machine.PWM class, to provide a
way to generate PWM output that is portable across the different ports.
Such functionality may already be available in one way or another (eg
through a Timer object), but because configuring PWM via a Timer is very
port-specific, and because it's a common thing to do, it's beneficial to
have a top-level construct for it.

The specification in this commit aims to provide core functionality in a
minimal way.  It also somewhat matches most existing ad-hoc implementations
of machine.PWM.

See discussion in adafruit#2283 and adafruit#4237.

Signed-off-by: Damien George <damien@micropython.org>

docs/library/machine.PWM.rst

@@ -0,0 +1,79 @@
+.. currentmodule:: machine
+.. _machine.PWM:
+
+class PWM -- pulse width modulation
+===================================
+
+This class provides pulse width modulation output.
+
+Example usage::
+
+    from machine import PWM
+
+    pwm = PWM(pin)          # create a PWM object on a pin
+    pwm.duty_u16(32768)     # set duty to 50%
+
+    # reinitialise with a period of 200us, duty of 5us
+    pwm.init(freq=5000, duty_ns=5000)
+
+    pwm.duty_ns(3000)       # set pulse width to 3us
+
+    pwm.deinit()
+
+Constructors
+------------
+
+.. class:: PWM(dest, \*, freq, duty_u16, duty_ns)
+
+   Construct and return a new PWM object using the following parameters:
+
+      - *dest* is the entity on which the PWM is output, which is usually a
+        :ref:`machine.Pin <machine.Pin>` object, but a port may allow other values,
+        like integers.
+      - *freq* should be an integer which sets the frequency in Hz for the
+        PWM cycle.
+      - *duty_u16* sets the duty cycle as a ratio ``duty_u16 / 65535``.
+      - *duty_ns* sets the pulse width in nanoseconds.
+
+   Setting *freq* may affect other PWM objects if the objects share the same
+   underlying PWM generator (this is hardware specific).
+   Only one of *duty_u16* and *duty_ns* should be specified at a time.
+
+Methods
+-------
+
+.. method:: PWM.init(\*, freq, duty_u16, duty_ns)
+
+   Modify settings for the PWM object.  See the above constructor for details
+   about the parameters.
+
+.. method:: PWM.deinit()
+
+   Disable the PWM output.
+
+.. method:: PWM.freq([value])
+
+   Get or set the current frequency of the PWM output.
+
+   With no arguments the frequency in Hz is returned.
+
+   With a single *value* argument the frequency is set to that value in Hz.  The
+   method may raise a ``ValueError`` if the frequency is outside the valid range.
+
+.. method:: PWM.duty_u16([value])
+
+   Get or set the current duty cycle of the PWM output, as an unsigned 16-bit
+   value in the range 0 to 65535 inclusive.
+
+   With no arguments the duty cycle is returned.
+
+   With a single *value* argument the duty cycle is set to that value, measured
+   as the ratio ``value / 65535``.
+
+.. method:: PWM.duty_ns([value])
+
+   Get or set the current pulse width of the PWM output, as a value in nanoseconds.
+
+   With no arguments the pulse width in nanoseconds is returned.
+
+   With a single *value* argument the pulse width is set to that value.

docs/library/machine.rst

@@ -167,6 +167,7 @@ Classes
    machine.Pin.rst
    machine.Signal.rst
    machine.ADC.rst
+   machine.PWM.rst
    machine.UART.rst
    machine.SPI.rst
    machine.I2C.rst