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diff --git a/source/faq.rst b/source/faq.rst index 13347f8..4b5b605 100644 --- a/source/faq.rst +++ b/source/faq.rst @@ -1,3 +1,5 @@ +.. highlight:: cpp + .. _faq: ================================== @@ -31,3 +33,63 @@ How do I replace ``PROGMEM``/put data into Flash? ------------------------------------------------- See :ref:`this note <arm-gcc-attribute-flash>`. + +How do I write to a pin at high speed? +-------------------------------------- + +Sometimes, :ref:`lang-digitalwrite` just isn't fast enough. If that's +your situation, you should first try using fast GPIO writes using the +low-level :ref:`libmaple-gpio` interface. This FAQ entry explains +how. + +You'll need to look up the :ref:`GPIO port and bit <gpio-ports>` which +correspond to the pin you want to write to. If you don't know what +that means, don't worry. We'll go through an example here. + +Let's say you want to write to pin 4 on the Maple. In order to find +out the port and bit number, take look at the Maple's :ref:`master pin +map <maple-pin-map-master>` next to "D4". You'll see that in the +"GPIO" column, there's "PB5". That's short for "**P**\ ort **B**, bit +**5**". So the GPIO port is "B", and the bit is "5". (If you're not +on the Maple, you can find your board's pin map :ref:`from here +<gpio-pin-maps>`). + +That's all you need to know. Now you can use the function +``gpio_write_bit()`` to quickly write to the pin. The way you call it +is by writing ``gpio_write_bit(GPIO<port>, <bit>, HIGH/LOW)``, where +``<port>`` is the GPIO port, ``<bit>`` is the bit, and ``HIGH`` or +``LOW`` is the level you want to write to the pin. Here's an example +program which writes pin 4 (GPIOB, bit 5) ``HIGH`` and then ``LOW`` +several times in a row each time it :ref:`lang-loop`\ s:: + + /* + Fast pin writing example, for Maple. + + This example works for pin 4 (PB5 on Maple). If you want to + use another pin (or are on another board), just change PIN, + PIN_PORT, and PIN_BIT as described above. + */ + + #define PIN 4 + #define PIN_PORT GPIOB + #define PIN_BIT 5 + + void setup() { + pinMode(PIN, OUTPUT); + } + + void loop() { + gpio_write_bit(PIN_PORT, PIN_BIT, HIGH); + gpio_write_bit(PIN_PORT, PIN_BIT, LOW); + gpio_write_bit(PIN_PORT, PIN_BIT, HIGH); + gpio_write_bit(PIN_PORT, PIN_BIT, LOW); + } + +Now, if you've already tried this and you still can't get enough +speed, there are some threads on the `forum`_ which might help you +squeeze a little extra out of your board. First, a `general summary +<http://forums.leaflabs.com/topic.php?id=860>`_ of other things to +try, with measurements of the speed you'll get. Next, a thread +featuring a `detailed discussion on pin capability +<http://forums.leaflabs.com/topic.php?id=774>`_, with a focus on +writes. |