.. _i2c: |i2c| ===== |i2c| is a crude and easy-to-hack serial protocol that requires only two wires/channels for communication between Maple and many other devices. .. contents:: Contents :local: Overview -------- Communication via |i2c| is broken up into messages. Every message is between a *master* device, which initiates the message, and a *slave* device, which responds. Slaves are addressed using 7-bit addresses (up to 127 unique devices); 10-bit addressing is also possible. 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. 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 (though slave mode is currently unimplemented). Hardware/Circuit Design ----------------------- .. FIXME [0.1.0] Link to board-specific values (BOARD_I2C1_SDA_PIN, etc.) Maple boards have two |i2c| ports. Maples reliably communicate 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 ` 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, but software for it is not yet implemented. .. _i2c-recommended-reading: Recommended Reading ------------------- * `I2C Bus `_ * `Wikipedia: I2C `_ * `Arduino I2C/TWI reference `_ * ST `Application note STM32F10xxx I2C optimized examples `_ (PDF)