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diff --git a/docs/source/pwm.rst b/docs/source/pwm.rst deleted file mode 100644 index 34ad508..0000000 --- a/docs/source/pwm.rst +++ /dev/null @@ -1,103 +0,0 @@ -.. _pwm: - -PWM -=== - -Pulse Width Modulation (PWM) 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 --------- - -.. FIXME [0.1.0] More information about how timer channels drive PWM - -Each PWM output is driven by an output channel 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. See your board's :ref:`pin mapping -tables <gpio-pin-maps>` to track down the correspondence -between timer channels and GPIO pins. - -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 -<lang-hardwaretimer-setprescalefactor>` (clock divider) in front of -the counter can increase this maximum period. Setting the -:ref:`period <lang-hardwaretimer-setperiod>` 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 ------------------- - -- :ref:`lang-pinmode` -- :ref:`lang-pwmwrite` -- :ref:`Timer API<lang-hardwaretimer>` (especially :ref:`setOverflow() - <lang-hardwaretimer-setoverflow>`, :ref:`setPrescaleFactor() - <lang-hardwaretimer-setprescalefactor>`, and :ref:`setPeriod() - <lang-hardwaretimer-setperiod>`). -- :ref:`Timers reference <timers>`. - -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: - - * `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) |