foumnd at http://www.smbus.org/specs/

1.5. Main Differences Between System Management Bus and I²C
The major differences between SMB and I²C fall into several categories including electrical,
timing, protocols, and operating modes.
· SMB is based on fixed voltage levels, the I²C levels are scaleable. However, the SMB logic
levels are easily met using standard 5 volt components.
· SMB specifies a minimum operational clock speed.
· SMB specifies device timeouts.
· SMB allows a slave device to stretch the cumulative clock (low) time, in a single message,
up to TLOW:SEXT. This allows, for example, a low-power microprocessor-based slave device,
such as a Smart Battery, sufficient time to “wake-up” and/or “marshal data.”
· SMB allows a master device to stretch the cumulative clock (low) time, in any single byte, up
to TLOW:MEXT. This allows, for example, a keyboard controller-based SMB emulation
sufficient time to service keyboard interrupts while hosting the SMBus.
· SMB specifies the protocol that an SMB device is allowed to use when communicating with
the SMB Host operating as a slave device.