System Management Bus (SMBus)

Overview 

System Management Bus (SMBus) is derived from I2C for communication with devices on the motherboard. A system may use SMBus to communicate with the peripherals on the motherboard without using dedicated control lines. SMBus peripherals can provide various manufacturer information, report errors, accept control parameters, etc.

Devices on the bus can operate in three roles: as a Controller that initiates transactions and controls the clock, a Peripheral that responds to transaction commands, or a Host, which is a specialized Controller, that provides the main interface to the system’s CPU. Zephyr has API for the Controller role.

SMBus peripheral devices can initiate communication with Controller with two methods:

Host Notify protocol: Peripheral device that supports the Host Notify protocol behaves as a Controller to perform the notification. It writes a three-bytes message to a special address “SMBus Host (0x08)” with own address and two bytes of relevant data.
SMBALERT# signal: Peripheral device uses special signal SMBALERT# to request attention from the Controller. The Controller needs to read one byte from the special “SMBus Alert Response Address (ARA) (0x0c)”. The peripheral device responds with a data byte containing its own address.

Currently, the API is based on SMBus Specification version 2.0

Note

See Rule A.2: Inclusive Language for information about the terminology used in this API.

SMBus Controller API 

Zephyr’s SMBus controller API is used when an SMBus device controls the bus, particularly the start and stop conditions and the clock. This is the most common mode used to interact with SMBus peripherals.