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.. _i2c:
|i2c|
=====
|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.
.. FIXME [Maple-specific values]
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
------------------
Currently, only low-level support in :ref:`libmaple-i2c` exists. A
Wiring-style library is planned for a future release.
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.
.. _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:
* `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)
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