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Diffstat (limited to 'docs/source/pwm.rst')
| -rw-r--r-- | docs/source/pwm.rst | 88 |
1 files changed, 21 insertions, 67 deletions
diff --git a/docs/source/pwm.rst b/docs/source/pwm.rst index 178d103..1a8f4df 100644 --- a/docs/source/pwm.rst +++ b/docs/source/pwm.rst @@ -1,10 +1,9 @@ .. _pwm: -============================== - Pulse-Width Modulation (PWM) -============================== +PWM +=== -Pulse Width Modulation is a basic technique to create repeated square +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 @@ -29,9 +28,9 @@ 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. +The following table shows which timer can generate 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: @@ -42,7 +41,7 @@ to track down exactly which timer *channel* corresponds to each pin. Timer1 | D6,D7,D8 Timer2 | D0,D1,D2,D3 Timer3 | D11,D12,D27,D28 - Timer4 | D5,D9,D14,D24 + Timer4 | D5,D9,D14,D24 Background ---------- @@ -66,12 +65,13 @@ 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. +<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): @@ -94,58 +94,13 @@ notoriously variable, especially the lower cost models): 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: - -``Timer1.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. +- :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 ------------------- @@ -159,6 +114,5 @@ Recommended Reading * `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) |