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Diffstat (limited to 'docs/source/libs/servo.rst')
| -rw-r--r-- | docs/source/libs/servo.rst | 61 |
1 files changed, 57 insertions, 4 deletions
diff --git a/docs/source/libs/servo.rst b/docs/source/libs/servo.rst index f92fd91..891f151 100644 --- a/docs/source/libs/servo.rst +++ b/docs/source/libs/servo.rst @@ -2,9 +2,10 @@ .. _libs-servo: -======= - Servo -======= +Servo +===== + +.. FIXME [0.0.10] this is out of date This documents the Servo library for controlling RC servomotors. It is implemented as a thin layer over the built-in :ref:`timer @@ -14,7 +15,8 @@ You can use this library in the :ref:`IDE <ide>` by choosing the Servo item under the Sketch > Import Library... menu. If you are using the :ref:`Unix toolchain <unix-toolchain>`, the -library is located in ``$LIB_MAPLE_HOME/libraries/Servo/``. +library is located in the ``/libraries/Servo/`` :ref:`libmaple` +directory. Servo Class Reference --------------------- @@ -106,3 +108,54 @@ servomotor attached to pin 9, you could write :: microseconds. This will be clamped to lie in the [``min``, ``max``\ ] pulse width range set during :ref:`attach() <libs-servo-attach>`. + +Arduino Compatibility +--------------------- + +The Servo class provides a public interface identical to the Arduino +version's documented functionality (as of Arduino 0021), so in most +cases, this library will be a drop-in replacement. + +However, there are some differences, essentially at the level of +implementation details. + +The major difference is that while the Arduino implementation drives +the servos with "bit-banged" :ref:`PWM <pwm>`, the Maple +implementation uses :ref:`timers <timers>` to drive the PWM directly. + +Consequently, **the Maple implementation only allows Servo instances +to** :ref:`attach <libs-servo-attach>` **to pins that support PWM**. + +To determine if a pin supports PWM (15 Maple pins do), you can either +check if "PWM" appears next to its number on the Maple silkscreen, or +consult the :ref:`pwmWrite() <lang-pwmwrite>` documentation. + +RC Servos expect a pulse approximately every 20ms. In the Maple +implementation, :ref:`periods <lang-hardwaretimer-setperiod>` are set +for entire timers, rather than individual channels. Thus, +``attach()``\ ing a Servo to a pin can interfere with other pins +associated with the same timer\ [#fard-servo]_. + +Because of this, we recommend connecting multiple servomotors to pins +which share a timer, in order to keep as many timers free for other +purposes as possible. Consult your board's :ref:`Timer Pin Map +<gpio-pin-maps>` to match up pins and timer channels. + +Another difference: although it is not publicly documented to do so, +the Arduino implementation of `attach() +<http://arduino.cc/en/Reference/ServoAttach>`_ returns the timer +channel associated with the newly-attached pin, or 0 on failure (as of +Arduino 0021). The Maple implementation returns :ref:`true +<lang-constants-true>` on success, and :ref:`false +<lang-constants-false>` on failure (and this is its documented +behavior). + +We currently provide a soft (bit-banged) implementation of the +:ref:`Wire <libs-wire>` I2C library. + +.. rubric:: Footnotes + +.. [#fard-servo] The Arduino implementation also captures timer + channels in groups as more Servo objects are attached, but the + details of which channels have their periods reset when are + slightly different. |