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diff --git a/docs/source/i2c.rst b/docs/source/i2c.rst
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--- a/docs/source/i2c.rst
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@@ -4,4 +4,89 @@
 |i2c|
 =====
 
-Stub.
+.. note::
+
+   The i2c interface is currently only available from the 'i2c' branch
+   of the github `libmaple <http://github.com/leaflabs/libmaple>`_
+   repository.
+
+|i2c| is a crude and easy-to-hack serial protocol that requires only
+two wires/channels for communication between many devices.  Every
+message passed on the bus is between a *master* (who initiates the
+message) and a *slave* device.  Slaves are addressed using 7-bit
+addresses (up to 127 unique devices); 10-bit addressing is possible,
+but currently unimplemented.  Every message consists of an arbitrary
+combination of 8-bit reads and writes as requested by the master.
+Higher level functionality, such as reading a particular register
+value, is achieved by writing to set the memory location then reading
+to pull out the data.
+
+Note that the master/slave designation is on a message-by-message
+basis. The Maple can act as both a master (messages initiated by user
+code) and slave device (responding to requests via configurable
+interrupt handlers) at the same time.
+
+.. contents:: Contents
+   :local:
+
+Hardware/Circuit Design
+-----------------------
+
+The Maple has two |i2c| ports.  Port 1 (i2c1) has SDA on header D9 and
+SCL on D5; Port 2 (i2c2) has SDA on D30 and SCL on D29.
+
+The Maple reliably communicates with up to a 400kHz clock speed; this
+doesn't translate into a 400kbps data rate except in extreme cases
+because of addressing and protocol overhead. We have tested clock
+speeds up to a megahertz and have had mixed results; in theory it
+could be possible to achieve even higher rates, but signal quality
+degrades rapidly and the bus becomes unreliable.
+
+Proper wiring and pull-up resistor selection are essential when
+incorporating |i2c| into a circuit, especially with data rates above
+100kHz. In the lab, we usually use approximately 5kΩ resistors with
+|vcc| (3.3V) as the high voltage, and try to connect the pullup
+voltage as close to the SDA and SCL pins as possible.  We recommend
+looking at the ST reference website for |i2c| (see the
+:ref:`recommended reading <i2c-recommended-reading>` below), starting
+with a slow clock rate (10kHz), and, if possible, using an
+oscilloscope to debug any issues.
+
+Function Reference
+------------------
+
+The function API for |i2c| is not finished! See the `source code
+<http://github.com/leaflabs/libmaple/blob/i2c/libmaple/i2c.h>`_ for
+now.
+
+.. TODO link to libmaple I2C docs once (1) finished, (2) in master
+
+SMBus
+-----
+
+The STM32 microcontroller has hardware support for SMBus; we simply
+have not written software for it. The SMBAL line for i2c1 is on header
+D4 and for i2c2 is on D31.
+
+.. TODO link to libmaple SMBus docs once (1) finished, (2) in master
+
+.. _i2c-recommended-reading:
+
+Recommended Reading
+-------------------
+
+* `i2c-bus.org <http://www.i2c-bus.org/>`_
+* `Wikipedia Article on i2c <http://en.wikipedia.org/wiki/I%C2%B2C>`_
+* `Arduino i2c/TWI reference <http://www.arduino.cc/playground/Learning/I2C>`_
+* 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)
+  * `Application Note on Advanced I2C Usage
+    <http://www.st.com/stonline/products/literature/an/15021.pdf>`_
+    (pdf)