SMBus Communication Protocol

SMBus is an acronym for System Management Bus. The SMBus is a derivative of the I2C bus, however there are enough changes that it warrants it’s own page.

Terminology

Term	Description
ACPI	ACPI is an initialism for Advanced Configuration And Power Interface.
CMI	CMI is an initialism for SMBus Control Method Interface.

History

SMBus was originally created by Intel in 1995.

Official Specifications

The official specifications for the SMBus can be found at http://smbus.org/specs/.

Comparison With I2C

Similarities

• Both use pull-up resistors (or constant-current sources) with an open-drain drive methodology.
• Both use the names SCL and SDA to the clock and data lines, respectively.
• Both use a 7-bit address with a 1-bit read/write bit to determine what slave to communicate with.
• Most I2C devices can communicate with SMBus devices (see the section Compatibility With I2C).

Differences

• The SMBus operates at frequencies between 10-100kHz, while the I2C bus can reach into the MHz.
• SMBus devices are designed to that no leakage current occurs through SDA and SCL when the devices power is turned off.

Compatibility With I2C

I2C and SMBus devices can communicate with each other in most situations. However, both devices have to operate within 10-100kHz.