merge libmaple docs ("leaflabs-docs") into ./docs

In the past, libample documentation was forked out of this repository
because the documentation had increased in scope. For the librambutan,
and the rambutan project in general, we will try to keep documentation
closer to the source code, so the librambutan-specific documentation
should live here. Other sections of leaflabs-docs will be culled in a
following commit.

This merge attempts to maintain history by using a subtree strategy.
Followed directions at:

    http://nuclearsquid.com/writings/subtree-merging-and-you/

Full history for files should be accessible using the "--follow" flag to
git log, eg:

    git log --follow docs/source/adc.rst

It should be possible to pull patches from leaflabs-docs with:

    git pull -s subtree leaflabs-docs master

... at least until the docs in this repository diverge significantly.

1 files changed, 62 insertions, 0 deletions

diff --git a/docs/source/usart.rst b/docs/source/usart.rst
new file mode 100644
index 0000000..dbbc81c
--- /dev/null
+++ b/docs/source/usart.rst
@@ -0,0 +1,62 @@
+.. _usart:
+
+USART
+=====
+
+A USART is more commonly known a *serial port*.  It's used to transmit
+information between Maple and another device (like your computer,
+another Maple, etc.).
+
+.. contents:: Contents
+   :local:
+
+.. _usart-circuit:
+
+Hardware/Circuit Design
+-----------------------
+
+To use a serial port to communicate with an external serial device,
+connect the TX pin to your device's RX pin, the RX to your device's TX
+pin, and your Maple board's ground to your device's ground.
+
+.. warning:: Don't connect these pins directly to an RS232 serial
+   port; they operate at +/- 12V and can damage your board.
+
+Each LeafLabs board has at least 3 separate USART devices. In the most
+simple use case, the RX and TX pins are used to send data at a
+predetermined speed (or *baud rate*).  Their usage is documented in
+the :ref:`Serial Ports <lang-serial>` language reference page.
+
+Which pins correspond to the USART TX and RX pins are given on your
+board's silkscreen, and also in the board-specific USART pin maps
+available here:
+
+* :ref:`Maple <maple-usart-map>`
+* :ref:`Maple RET6 Edition <maple-ret6-usart-map>`
+* :ref:`Maple Mini <maple-mini-usart-map>`
+* :ref:`Maple Native Beta <maple-native-b-usart-map>`
+
+If you use a particular serial port, you cannot also use its
+communication pins for other purposes at the same time.
+
+Compatible Devices and Specifications
+-------------------------------------
+
+We have successfully used the USART ports with an FT232R-based
+USB-serial converter at up to 115200 baud.  Higher speeds should
+certainly be possible.
+
+Recommended Reading
+-------------------
+
+* `Wikipedia article on Universal Asynchronous Receiver/Transmitter
+  (USART)
+  <http://en.wikipedia.org/wiki/Universal_asynchronous_receiver/transmitter>`_
+* `Arduino Serial reference
+  <http://arduino.cc/en/Reference/Serial>`_
+* ST `Reference Manual RM0008
+  <http://www.st.com/web/en/resource/technical/document/reference_manual/CD00171190.pdf>`_
+  (PDF), Chapter 27.
+* `Serial and UART Tutorial
+  <http://www.freebsd.org/doc/en/articles/serial-uart/>`_ (written for
+  FreeBSD, but contains a large amount of general information).