docs/library/machine: Add docs for Counter and Encoder.

Add documentation for `machine.Counter` and `machine.Encoder` as currently
implemented by the esp32 port, but intended to be implemented by other
ports.

Originally authored by: Ihor Nehrutsa <Ihor.Nehrutsa@gmail.com> and
Jonathan Hogg <me@jonathanhogg.com>.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

docs/esp32/quickref.rst

@@ -587,6 +587,21 @@ Use the :ref:`esp32.PCNT <esp32.PCNT>` class::
 The PCNT hardware supports monitoring multiple pins in a single unit to
 implement quadrature decoding or up/down signal counters.
 
+See the :ref:`machine.Counter <machine.Counter>` and
+:ref:`machine.Encoder <machine.Encoder>` classes for simpler abstractions of
+common pulse counting applications::
+
+    from machine import Pin, Counter
+
+    counter = Counter(0, Pin(2))    # create a counter as above and start it
+    count = counter.value()         # read the count as an arbitrary precision signed integer
+
+    encoder = Encoder(0, Pin(12), Pin(14))    # create an encoder and begin counting
+    count = encoder.value()                   # read the count as an arbitrary precision signed integer
+
+Note that the id passed to these ``Counter()`` and ``Encoder()`` objects must be
+a PCNT id.
+
 Software SPI bus
 ----------------
 

docs/library/esp32.rst

@@ -336,6 +336,9 @@ implemented as thin Python shims around :class:`PCNT`.
     value. Thus the ``IRQ_ZERO`` event will also trigger when either of these
     events occurs.
 
+See the :ref:`machine.Counter <machine.Counter>` and
+:ref:`machine.Encoder <machine.Encoder>` classes for simpler abstractions of
+common pulse counting applications.
 
 .. _esp32.RMT:
 

docs/library/machine.Counter.rst

@@ -0,0 +1,93 @@
+.. currentmodule:: machine
+.. _machine.Counter:
+
+class Counter -- pulse counter
+==============================
+
+Counter implements pulse counting by monitoring an input signal and counting
+rising or falling edges.
+
+Minimal example usage::
+
+    from machine import Pin, Counter
+
+    counter = Counter(0, Pin(0, Pin.IN))  # create Counter for pin 0 and begin counting
+    value = counter.value()               # retrieve current pulse count
+
+Availability: **ESP32**
+
+Constructors
+------------
+
+.. class:: Counter(id, ...)
+
+   Returns the singleton Counter object for the the given *id*. Values of *id*
+   depend on a particular port and its hardware. Values 0, 1, etc. are commonly
+   used to select hardware block #0, #1, etc.
+
+   Additional arguments are passed to the :meth:`init` method described below,
+   and will cause the Counter instance to be re-initialised and reset.
+
+   On ESP32, the *id* corresponds to a :ref:`PCNT unit <esp32.PCNT>`.
+
+Methods
+-------
+
+.. method:: Counter.init(src, *, ...)
+
+   Initialise and reset the Counter with the given parameters:
+
+   - *src* specifies the input pin as a :ref:`machine.Pin <machine.Pin>` object.
+     May be omitted on ports that have a predefined pin for a given hardware
+     block.
+
+   Additional keyword-only parameters that may be supported by a port are:
+
+   - *edge* specifies the edge to count. Either ``Counter.RISING`` (the default)
+     or ``Counter.FALLING``. *(Supported on ESP32)*
+
+   - *direction* specifies the direction to count. Either ``Counter.UP`` (the
+     default) or ``Counter.DOWN``. *(Supported on ESP32)*
+
+   - *filter_ns* specifies a minimum period of time in nanoseconds that the
+     source signal needs to be stable for a pulse to be counted. Implementations
+     should use the longest filter supported by the hardware that is less than
+     or equal to this value. The default is 0 (no filter). *(Supported on ESP32)*
+
+.. method:: Counter.deinit()
+
+   Stops the Counter, disabling any interrupts and releasing hardware resources.
+   A Soft Reset should deinitialize all Counter objects.
+
+.. method:: Counter.value([value])
+
+   Get, and optionally set, the counter value as a signed integer.
+   Implementations must aim to do the get and set atomically (i.e. without
+   leading to skipped counts).
+
+   This counter value could exceed the range of a :term:`small integer`, which
+   means that calling :meth:`Counter.value` could cause a heap allocation, but
+   implementations should aim to ensure that internal state only uses small
+   integers and therefore will not allocate until the user calls
+   :meth:`Counter.value`.
+
+   For example, on ESP32, the internal state counts overflows of the hardware
+   counter (every 32000 counts), which means that it will not exceed the small
+   integer range until ``2**30 * 32000`` counts (slightly over 1 year at 1MHz).
+
+   In general, it is recommended that you should use ``Counter.value(0)`` to reset
+   the counter (i.e. to measure the counts since the last call), and this will
+   avoid this problem.
+
+Constants
+---------
+
+.. data:: Counter.RISING
+          Counter.FALLING
+
+   Select the pulse edge.
+
+.. data:: Counter.UP
+          Counter.DOWN
+
+   Select the counting direction.

docs/library/machine.Encoder.rst

@@ -0,0 +1,72 @@
+.. currentmodule:: machine
+.. _machine.Encoder:
+
+class Encoder -- quadrature decoding
+====================================
+
+Encoder implements decoding of quadrature signals as commonly output from
+rotary encoders, by counting either up or down depending on the order of two
+input pulses.
+
+Minimal example usage::
+
+    from machine import Pin, Encoder
+
+    counter = Counter(0, Pin(0, Pin.IN), Pin(1, Pin.IN))   # create Encoder for pins 0, 1 and begin counting
+    value = counter.value()                                # retrieve current count
+
+Availability: **ESP32**
+
+Constructors
+------------
+
+.. class:: Encoder(id, ...)
+
+   Returns the singleton Encoder object for the the given *id*. Values of *id*
+   depend on a particular port and its hardware. Values 0, 1, etc. are commonly
+   used to select hardware block #0, #1, etc.
+
+   Additional arguments are passed to the :meth:`init` method described below,
+   and will cause the Encoder instance to be re-initialised and reset.
+
+   On ESP32, the *id* corresponds to a :ref:`PCNT unit <esp32.PCNT>`.
+
+Methods
+-------
+
+.. method:: Encoder.init(phase_a, phase_b, *, ...)
+
+   Initialise and reset the Encoder with the given parameters:
+
+   - *phase_a* specifies the first input pin as a
+     :ref:`machine.Pin <machine.Pin>` object.
+
+   - *phase_b* specifies the second input pin as a
+     :ref:`machine.Pin <machine.Pin>` object.
+
+   These pins may be omitted on ports that have predefined pins for a given
+   hardware block.
+
+   Additional keyword-only parameters that may be supported by a port are:
+
+   - *filter_ns* specifies a minimum period of time in nanoseconds that the
+     source signal needs to be stable for a pulse to be counted. Implementations
+     should use the longest filter supported by the hardware that is less than
+     or equal to this value. The default is 0 (no filter). *(Supported on ESP32)*
+
+   - *phases* specifies the number of signal edges to count and thus the
+     granularity of the decoding. e.g. 4 phases corresponds to "4x quadrature
+     decoding", and will result in four counts per pulse. Ports may support
+     either 1, 2, or 4 phases and the default is 1 phase. *(Supported on ESP32)*
+
+.. method:: Encoder.deinit()
+
+   Stops the Encoder, disabling any interrupts and releasing hardware resources.
+   A Soft Reset should deinitialize all Encoder objects.
+
+.. method:: Encoder.value([value])
+
+   Get, and optionally set, the encoder value as a signed integer.
+   Implementations should aim to do the get and set atomically.
+
+   See :meth:`machine.Counter.value` for details about overflow of this value.

docs/library/machine.rst

@@ -268,6 +268,8 @@ Classes
    machine.I2S.rst
    machine.RTC.rst
    machine.Timer.rst
+   machine.Counter.rst
+   machine.Encoder.rst
    machine.WDT.rst
    machine.SD.rst
    machine.SDCard.rst

docs/reference/glossary.rst

@@ -188,6 +188,13 @@ Glossary
         Most MicroPython boards make a REPL available over a UART, and this is
         typically accessible on a host PC via USB.
 
+    small integer
+        MicroPython optimises the internal representation of integers such that
+        "small" values do not take up space on the heap, and calculations with
+        them do not require heap allocation. On most 32-bit ports, this
+        corresponds to values in the interval ``-2**30 <= x < 2**30``, but this
+        should be considered an implementation detail and not relied upon.
+
     stream
         Also known as a "file-like object". A Python object which provides
         sequential read-write access to the underlying data. A stream object