path: root/share/man/man4/iicbus.4

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.Dd November 17, 2014
.Dt IICBUS 4
.Os
.Sh NAME
.Nm iicbus
.Nd I2C bus system
.Sh SYNOPSIS
.Cd "device iicbus"
.Cd "device iicbb"
.Pp
.Cd "device iic"
.Cd "device ic"
.Cd "device iicsmb"
.Sh DESCRIPTION
The
.Em iicbus
system provides a uniform, modular and architecture-independent
system for the implementation of drivers to control various I2C devices
and to utilize different I2C controllers.
.Sh I2C
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.
.Pp
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.
.Pp
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 functionality.
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.
.Pp
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.
.Sh DEVICES
Some I2C device drivers are available:
.Pp
.Bl -column "Device drivers" -compact
.It Em Devices Ta Em Description
.It Sy iic Ta "general i/o operation"
.It Sy ic Ta "network IP interface"
.It Sy iicsmb Ta "I2C to SMB software bridge"
.El
.Sh INTERFACES
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.
.Pp
I2C interfaces may act on the bus as slave devices, allowing spontaneous
bidirectional communications, thanks to the multi-master capabilities of the
I2C protocol.
.Pp
Some I2C interfaces are available:
.Pp
.Bl -column "Interface drivers" -compact
.It Em Interface Ta Em Description
.It Sy pcf Ta "Philips PCF8584 master/slave interface"
.It Sy iicbb Ta "generic bit-banging master-only driver"
.It Sy lpbb Ta "parallel port specific bit-banging interface"
.It Sy bktr Ta "Brooktree848 video chipset, hardware and software master-only interface"
.El
.Sh BUS FREQUENCY CONFIGURATION
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.  
.Pp
When a system supports multiple I2C busses, a different frequency can
be configured for each bus by number, represented by the
.Va %d
in the variable names below.
Busses can be configured using any combination of device hints,
Flattened Device Tree (FDT) data, tunables set via
.Xr loader 8 ,
or at runtime using
.Xr sysctl 8 .
When configuration is supplied using more than one method, FDT and
hint data will be overridden by a tunable, which can be overriden by
.Xr sysctl 8 .
.Ss Device Hints
Set
.Va 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 systems that use FDT data if
no frequency is configured using FDT.
.Ss Flattened Device Tree Data
Configure the I2C bus speed using the FDT standard
.Va clock-frequency
property of the node describing the I2C controller hardware.
.Ss Sysctl and Tunable
Set
.Va dev.iicbus.%d.frequency
in
.Xr loader.conf 5 .
The same variable can be changed at any time with
.Xr sysctl 8 .
Reset the bus using
.Xr i2c 8
or the
.Xr iic 4 
.Va I2CRSTCARD
ioctl to make the change take effect.
.Sh SEE ALSO
.Xr fdt 4 ,
.Xr iic 4 ,
.Xr iicbb 4 ,
.Xr lpbb 4 ,
.Xr pcf 4 ,
.Xr i2c 8
.Sh HISTORY
The
.Nm
manual page first appeared in
.Fx 3.0 .
.Sh AUTHORS
This
manual page was written by
.An Nicolas Souchu .