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.. _pwm:
==============================
Pulse-Width Modulation (PWM)
==============================
Pulse Width Modulation is a basic technique to create repeated square
waves (digital high/low voltage transitions) of user defined length
and duty cycle. It can be used as a way to encode an "analog" signal
on a single digital (high/low) line using the time between transitions
("pulse width") as the variable; this technique is commonly used to
send servo position and motor speed commands. Another use is to use to
the ratio of "high" and "low" time to approximate a voltage output;
this technique can be used to dim an LED or even (with careful
filtering) generate audio waveforms.
.. contents:: Contents
:local:
Overview
--------
The Maple has a large number of 16-bit PWM outputs, each connected to
one of 4 timers. Some configuration, such as the clock rate or
prescaling, must be common to the entire timer; see the :ref:`timer
documentation <timers>` for more information.
Note that unlike the Arduino, the Maple does not have PWM
functionality on pin D10; all other pins are :ref:`compatible
<compatibility>`.
The following table shows which :ref:`timer <timers>` generates which
PWM outputs. See the :ref:`pin mapping table <pin-mapping-mega-table>`
to track down exactly which timer *channel* corresponds to each pin.
.. _pwm-timer-table:
.. csv-table::
:header: Timer, PWM Headers
:delim: |
Timer1 | D6,D7,D8
Timer2 | D0,D1,D2,D3
Timer3 | D11,D12,D27,D28
Timer4 | D5,D9,D14,D24
Background
----------
In its simplest form, the device is a single counter with two
variables. The counter starts at zero, and the output starts at
"high". The counter increments every clock cycle until it reaches the
first variable number, at which point the output goes "low". The
counter continues incrementing until it reaches the second variable at
which point the output goes "high" again and the counter resets to
zero. The time spent with output high is called the **pulse duration**
or **duty**; the total time before repeat is the **period**.
This simple functionality could be approximated in software by setting
a GPIO high or low, but the beauty of PWM is that user code simply has
to configure the device and set the two variables and the device will
function on its own; no further microprocessor cycles will be
consumed, and a repeated high/low waveform will spew out.
The Maple has 16-bit PWM resolution, which means that the counter and
variables can be as large as 65535, as opposed to 255 with 8-bit
resolution. With a 72MHz clock rate, a PWM output could have maximum
period of about one millisecond; using a :ref:`prescaler
<pwm-prescaler>` (clock divider) in front of the counter can increase
this maximum period. Setting the :ref:`period <pwm-overflow>` to
something other than the maximum value gives further control over the
total length of the waveform. However, this effectively limits the
resolution with which the duty can be modified: the duty must be less
than or equal to the period.
Here are some commonly used PWM configurations (note that servos are
notoriously variable, especially the lower cost models):
+-------------+----------+-----------+---------+---------------+------+
|**Purpose** |**Period**|**Duty** |Prescaler|Period |Duty |
| |(ms) |(ms) | | | |
+=============+==========+===========+=========+===============+======+
|LED throb |0.020 |0--0.020 |1 (none) |65535 (default)|0--767|
| | | | | | |
+-------------+----------+-----------+---------+---------------+------+
|Servo control|20 |1.25 (0°) |21 |65535 (default)|4096 |
| | | | | | |
| | |1.50 (90°) |21 |65535 (default)|4915 |
| | | | | | |
| | |1.75 (180°)|21 |65535 (default)|5734 |
| | | | | | |
+-------------+----------+-----------+---------+---------------+------+
Function Reference
------------------
``pinMode(pin_num, PWM)``
This command is usually called from `setup()`_ to tell the
microcontroller that pin_num should be configured to PWM
output. ``PWM`` implies regular driven OUTPUT; ``PWM_OPEN_DRAIN`` is
also available (see the list of :ref:`GPIO modes <gpio-modes>` for
more information).
.. _pwm-pwmwrite:
``pwmWrite(pin_num, value)``
This command sets the PWM duty. User code is expected to determine
and honor the maximum value (based on the configured period). As a
convenience, ``analogWrite`` is an alias for ``pwmWrite`` to ease
porting Arduino code, though period and duty will have to be
recalibrated (see :ref:`compatibility <compatibility>`).
.. _pwm-overflow:
``Timer[1,2,3,4].setOverflow(overflow)``
This function sets the period ("reload" or "overflow") value for
an entire PWM timer bank. The value is 16bit (0 to 65535) and
determines the maximum value that can be written with
:ref:`pwmWrite() <pwm-pwmwrite>` corresponding to 100% duty
cycle. This also affects the PWM frequency: the higher reload is,
the lower the PWM frequency will be.
The PWM output pin starts HIGH, then the timer begins counting up
from zero (with frequency equal to 72MHz/:ref:`prescaler
<pwm-prescaler>`) until it hits the duty value, at which point it
drops to LOW. The timer then continues counting up until it hits
the total period (set with this function), at which point the
cycle starts again.
.. _pwm-prescaler:
``Timer[1,2,3,4].setPrescaleFactor(prescale)``
Find the appropriate timer for a given PWM header using the table
:ref:`above <pwm-timer-table>`, then set the prescaler. A
prescaler is a clock divider. The timer will normally count with
frequency equal to the STM32's normal clock (72MHz); this
corresponds to setting ``prescale`` to 1 (which is the default).
If a longer frequency is desired, use a larger ``prescale`` value.
For instance, an 8MHz frequency can be achieved by setting
``prescale`` to 9, since 72MHz / 9 = 8MHz.
This function is normally called once from, `setup()`_, but the
timer can be reconfigured with a new prescaler at any time.
* Configure the prescaler and overflow values to generate a timer
* reload with a period as close to the given number of
* microseconds as possible.
*
* The return value is the overflow, which may be used to set
* channel compare values. However, if a clock that fires an
* interrupt every given number of microseconds is all that is
* desired, and the relative "phases" are unimportant, channel
* compare values may all be set to 1.
``Timer[1,2,3,4].setPeriod(period_in_microseconds)``
Configure the prescaler and overflow values to generate a timer
reload with a period as close to the given number of microseconds
as possible.
The return value is the overflow, which may be used to set channel
compare values. However, if a clock that fires an interrupt every
given number of microseconds is all that is desired, and the
relative "phases" are unimportant, channel compare values may all
be set to 1.
Recommended Reading
-------------------
* `Wikipedia Article on Pulse-width modulation
<http://en.wikipedia.org/wiki/Pulse-width_modulation>`_
* `Arduino tutorial on PWM <http://www.arduino.cc/en/Tutorial/PWM>`_
* `Secrets of Arduino PWM
<http://www.arcfn.com/2009/07/secrets-of-arduino-pwm.html>`_ by Ken
Shirriff
* `So You Want To Use PWM, Eh? <http://www.arcfn.com/2009/07/secrets-of-arduino-pwm.html>`_ at Non-Lexical Vocables
* STMicro documentation for STM32F103RB microcontroller:
* `All <http://www.st.com/mcu/devicedocs-STM32F103RB-110.html>`_
* `Datasheet <http://www.st.com/stonline/products/literature/ds/13587.pdf>`_ (pdf)
* `Reference Manual <http://www.st.com/stonline/products/literature/rm/13902.pdf>`_ (pdf)
|
