NAME

	i2c – I²C Bus Interface

SYNOPSIS

	bind –a #J /dev /dev/bus/i2c.n.ctl /dev/bus/i2c.n.data

DESCRIPTION

The I²C (Inter–Integrated Circuit) bus is a serial bus standard invented by Philips. It is used to connect chips over short–distances like on a motherboard or between boards. The bus requires only a single data signal SDA, a clock signal SDC and ground. Devices on the bus have 7–bit or 10–bit addresses, which is determined by the device (see the datasheet). Every device monitors the bus and waits for the master to initiate a transaction. Each transaction starts with the master sending the target device address with a direction bit and the device acknowledges each byte received if its address matched. Depending on the device, more bytes must be transmitted after the device–address. For example, encoding a register number at a memory offset. We refer to these bytes as the sub–address. After the address phase, the data phase begins where zero or more data bytes are either transmitted or received depending on the transaction direction. This driver supports multiple buses (controllers) to be registered by platform specific code in the kernel. Each bus is represented as a directory with a platform specific name. Because all received bytes are acknowledged by target devices, it is possible for the master to enumerate all existing addresses on a bus, which is done when accessing the bus directory. Platform specific code can also register individual devices, supplying a meaningful device configuration. It can also be necessary to manually register devices if the device has been disabled by an enable signal or otherwise inhibited during enumeration. Each device appears as a control ( i2c.n.ctl ) and a data file ( i2c.n.data ) in the bus directory where n is the device–address in hexadecimal. The data file is used to initiate transactions to the device. The initial sub–address bytes sent are derived from the file position according to the device configuration, and the length of the data phase is controlled by the requested read or write size. The requested size might be truncated by the driver if the request exceeds the logical file–size. The control–file provides access to the device configuration. When read, it reproduces this configuration in text form. The following text lines are accepted when written to the control–file: subaddress n   Sets the length n, in bytes, of the sub–address for the device that is derived from the file–position of the data–file. This can be set to zero if no sub–address bytes should be generated by the driver. By default, the sub–address length is 1. size n         Changes the logical size of the data–file to n. Making sure the file–position is not exceeding this size.

SOURCE

	/sys/src/9/port/devi2c.c

HISTORY

	The file–interface has been inspired by infernos devi2c, but with the addition of supporting multiple buses and device enumeration.