System Management Bus

System Management Bus

The System Management Bus (abbreviated to SMBus or SMB) is a simple two-wire bus, derived from I²C and used for communication with low-bandwidth devices on a motherboard, especially power related chips such as a laptop's rechargeable battery subsystem (see Smart Battery Data). Other devices might include temperature, fan, or voltage sensors; and lid switches. PCI add-in cards may connect to an SMBus segment.

A device can provide manufacturer information, indicate its model/part number, save its state for a suspend event, report different types of errors, accept control parameters, and return status. The SMBus is generally not user configurable or accessible. Although SMBus devices usually can't identify their functionality, a new PMBus coalition has extended SMBus to include conventions allowing that.

The SMBus was defined by Intel in 1995. It carries clock, data, and instructions and is based on Philips' I²C serial bus protocol. Its clock frequency range is 10 kHz to 100 kHz. (PMBus extends this to 400 kHz.) Its voltage levels and timings are more strictly defined than those of I²C, but devices belonging to the two systems are often successfully mixed on the same bus.

SMBus is mostly a subset of I²C; more loosely specified I²C devices may cause an SMBus to hang. The SMBus has an extra optional shared interrupt signal called SMBALERT#, which can be used by slaves to tell the host to ask its slaves about events of interest. SMBus also defines a less common "Host Notify Protocol", providing similar notifications but passing more data and building on the I²C multi-master mode.

FreeBSD, Linux, Windows 2000, Windows XP and Windows Vista support SMBus devices, but Windows 98 and earlier versions do not.

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