100 lines
3 KiB
Text
100 lines
3 KiB
Text
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.TH I2C 3
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.SH NAME
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i2c \- I²C Bus Interface
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.SH SYNOPSIS
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.nf
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.B bind -a #J /dev
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.PP
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.nf
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.BI /dev/ bus /i2c. n .ctl
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.BI /dev/ bus /i2c. n .data
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.fi
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.SH DESCRIPTION
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The I²C (Inter-Integrated Circuit) bus is a serial bus standard
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invented by Philips.
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It is used to connect chips over short-distances like on a motherboard
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or between boards.
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The bus requires only a single data signal
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.IR SDA ,
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a clock signal
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.I SDC
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and ground.
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Devices on the bus have 7-bit or 10-bit addreesses, which is
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determined by the device (see the datasheet).
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Every device monitors the bus and waits for the master to initiate
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a transaction.
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Each transaction starts with the master sending the target device address
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and direction bit and the device is acknowledging each byte
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received if its address matched.
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Depending on the device, more bytes must be transmitted after the
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.IR device-address .
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For example, to encode a register number an memory offset.
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We refer to these bytes as the
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.IR sub-address .
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After the address phase, the data phase begins where
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zero or more data bytes are eigther transmitted or received
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depending on the transactoin direction.
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.PP
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This driver supports multiple buses (controllers) to be registered
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by platform specific code in the kernel.
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Each bus is represented as a directory with a platform specific name.
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.PP
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Because all received bytes are acknowledged by target devices,
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it is possible for the master to enumerate all existing addresses
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on a bus, which is done when accessing the
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.I bus
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directory.
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.PP
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Platform specific code can also register individual devices,
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supplying a meaningfull device configuration.
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It can also be neccesary to manually register devices if
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the device has been disabled by a enable signal or
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otherwise inhibited during enumeration.
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.PP
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Each device appears as a control (
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.BI i2c. n .ctl
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)
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and a data file (
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.BI i2c. n .data
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) in the
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.I bus
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directory
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where
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.I n
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is the device-address in hexadecimal.
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.PP
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The data file is used to initiate transactions to the device.
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The initial
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.I sub-address
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bytes sent are derived from the file position according to the device
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configuration and the length of the data phase is controlled by
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the requested read or write size.
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The requested size might be truncated by the driver if
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the request exceeds the logical file-size.
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.PP
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The control-file provides access the device configuration.
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When read,
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it reproduces this configuration in text form.
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The following text lines are accepted when written to the control-file:
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.TF "subaddress n"
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.TP
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.BI "subaddress " n
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Sets the length
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.IR n ,
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in bytes, of the sub-address for the device
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that is derived from the file-position of the data-file.
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This can be set to zero if no sub-address bytes should
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be generated by the driver.
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By default, the sub-address length is 1.
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.TP
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.BI "size " n
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Changes the logical size of the data-file to
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.IR n .
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Making sure the file-position is not exceeding this size.
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.SH SOURCE
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.B /sys/src/9/port/devi2c.c
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.SH HISTORY
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The file-interface has been inspired by infernos devi2c,
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but with the addition of supporting multiple buses
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and device enumeration.
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