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IICBUS(4)	       FreeBSD Kernel Interfaces Manual		     IICBUS(4)

     iicbus -- I2C bus system

     device iicbus
     device iicbb

     device iic
     device ic
     device iicsmb

     The iicbus	system provides	a uniform, modular and architecture-indepen-
     dent system for the implementation	of drivers to control various I2C
     devices and to utilize different I2C controllers.

     I2C is an acronym for Inter Integrated Circuit bus.  The I2C bus was
     developed in the early 1980's by Philips semiconductors.  Its purpose was
     to	provide	an easy	way to connect a CPU to	peripheral chips in a TV-set.

     The BUS physically	consists of 2 active wires and a ground	connection.
     The active	wires, SDA and SCL, are	both bidirectional.  Where SDA is the
     Serial DAta line and SCL is the Serial CLock line.

     Every component hooked up to the bus has its own unique address whether
     it	is a CPU, LCD driver, memory, or complex function chip.	 Each of these
     chips can act as a	receiver and/or	transmitter depending on its function-
     ality.  Obviously an LCD driver is	only a receiver, while a memory	or I/O
     chip can both be transmitter and receiver.	 Furthermore there may be one
     or	more BUS MASTERs.

     The BUS MASTER is the chip	issuing	the commands on	the BUS.  In the I2C
     protocol specification it is stated that the IC that initiates a data
     transfer on the bus is considered the BUS MASTER.	At that	time all the
     others are	regarded to as the BUS SLAVEs.	As mentioned before, the IC
     bus is a Multi-MASTER BUS.	 This means that more than one IC capable of
     initiating	data transfer can be connected to it.

     Some I2C device drivers are available:

     Devices	       Description
     iic	       general i/o operation
     ic		       network IP interface
     iicsmb	       I2C to SMB software bridge

     The I2C protocol may be implemented by hardware or	software.  Software
     interfaces	rely on	very simple hardware, usually two lines	twiddled by 2
     registers.	 Hardware interfaces are more intelligent and receive 8-bit
     characters	they write to the bus according	to the I2C protocol.

     I2C interfaces may	act on the bus as slave	devices, allowing spontaneous
     bidirectional communications, thanks to the multi-master capabilities of
     the I2C protocol.

     Some I2C interfaces are available:

     Interface		  Description
     pcf		  Philips PCF8584 master/slave interface
     iicbb		  generic bit-banging master-only driver
     lpbb		  parallel port	specific bit-banging interface
     bktr		  Brooktree848 video chipset, hardware and software
			  master-only interface

     The operating frequency of	an I2C bus may be fixed	or configurable.  The
     bus may be	used as	part of	some larger standard interface,	and that
     interface specification may require a fixed frequency.  The driver	for
     that hardware would not honor an attempt to configure a different speed.
     A general purpose I2C bus,	such as	those found in many embedded systems,
     will often	support	multiple bus frequencies.

     When a system supports multiple I2C busses, a different frequency can be
     configured	for each bus by	number,	represented by the %d in the variable
     names below.  Busses can be configured using any combination of device
     hints, Flattened Device Tree (FDT)	data, tunables set via loader(8), or
     at	runtime	using sysctl(8).  When configuration is	supplied using more
     than one method, FDT and hint data	will be	overridden by a	tunable, which
     can be overridden by sysctl(8).

   Device Hints
     Set hint.iicbus.%d.frequency to the frequency in Hz, on systems that use
     device hints to configure I2C devices.  The hint is also honored by sys-
     tems that use FDT data if no frequency is configured using	FDT.

   Flattened Device Tree Data
     Configure the I2C bus speed using the FDT standard	clock-frequency	prop-
     erty of the node describing the I2C controller hardware.

   Sysctl and Tunable
     Set dev.iicbus.%d.frequency in loader.conf(5).  The same variable can be
     changed at	any time with sysctl(8).  Reset	the bus	using i2c(8) or	the
     iic(4) I2CRSTCARD ioctl to	make the change	take effect.

     bktr(4), fdt(4), iic(4), iicbb(4),	lpbb(4), pcf(4), i2c(8)

     The iicbus	manual page first appeared in FreeBSD 3.0.

     This manual page was written by Nicolas Souchu.

FreeBSD	11.1		       November	17, 2014		  FreeBSD 11.1


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