While SMBus is derived from I²C, there are several major differences between the specifications of the two busses in the areas of electricals, timing, protocols and operating modes.
Electrical Input Voltage (VIL and VIH) When mixing devices, the I²C specification defines the input levels to be 30% and 70% of the supply voltage
VDD, which may be 5 V, 3.3 V, or any other value. Instead of relating the bus input levels to
VDD, SMBus defines them to be fixed. SMBus 2.0 defines
VIL,max at 0.8 V and
VIH,min at 2.1 V, and supports a
VDD ranging from 3 to 5 V, while in SMBus 3.0, the levels are defined at 0.8 and 1.35 V, with a
VDD ranging from 1.8 to 5 V.
SMBALERT# 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. ==Support==