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diff --git a/docs/source/faq.rst b/docs/source/faq.rst new file mode 100644 index 0000000..bd155ae --- /dev/null +++ b/docs/source/faq.rst @@ -0,0 +1,115 @@ +.. highlight:: cpp + +.. _faq: + +================================== + Frequently Asked Questions (FAQ) +================================== + +.. contents:: Contents + :local: + +.. _faq-atoi: + +Can I use ``atoi()``, ``strol()``, etc.? +---------------------------------------- + +Just ``#include <stdlib.h>``. See :ref:`arm-gcc-libc` for more +information. + +.. _faq-dynamic-memory: + +Can I use ``malloc()``/``new``? +------------------------------- + +For ``malloc()``, just ``#include <stdlib.h>``, and everything should +work fine. Be careful, though! This isn't like C programming on a +PC. You're on the bare metal, and probably shouldn't be using dynamic +memory unless you know what you're doing. + +``new`` should work out of the box (the warning about knowing what +you're doing applies here as well). If you find that ``new`` (or +``new[]``, ``delete``, etc.) is broken in some way, that's a bug; +please let us know in the `forum`_. + +Some information on the heap: on boards with an external SRAM chip +(like Maple Native), the heap is located on external SRAM by default. +For other boards, the heap is located immediately after ``.bss``, and +grows towards the stack. (In all cases, statically allocated +variables and the stack are located on internal SRAM, for performance +reasons). + +.. _faq-flash-tables: + +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. And finally, `another thread +<http://forums.leaflabs.com/topic.php?id=895>`_ on the subject which +summarizes a variety of other threads on doing I/O quickly. + +Can I use Maple without Maple IDE? +---------------------------------- + +Yes. See :ref:`unix-toolchain` for the details. + |