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2.13 System Management Bus (SMBus)
The SMBus is primarily used as an interface to manage peripherals such as serial presence detect (SPD)
on RAM, thermal sensors, PCIe devices, smart battery, etc. The devices that can connect to the SMBus
can be located on the Module and Carrier. Designers need to take note of several implementation issues
to ensure reliable SMBus interface operation. The SMBus is similar to I2C. I2C devices have the
potential to lock up the data line while sending information and require a power cycle to clear the fault
condition. SMBus devices contain a timeout to monitor for and correct this condition. Designers are
urged to use SMBus devices when possible over standard I2C devices. COM Express Modules are
required to power SMBus devices from Early Power in order to have control during system states S0-S5.
The devices on the Carrier Board using the SMBus are normally powered by the 3.3V main power. To
avoid current leakage between the main power of the Carrier Board and the Suspend power of the
Module, the SMBus on the Carrier Board must be separated by a bus switch from the SMBus of the
Module. However, if the Carrier Board also uses Suspend powered SMBus devices that are designed to
operate during system states S3-S5, then these devices must be connected to the Suspend powered
side of the SMBus, i. e. between the COM Express Module and the bus switch. Since the SMBus is used
by the Module and Carrier, care must be taken to ensure that Carrier based devices do not overlap the
address space of Module based devices. Typical Module located SMBus devices and their addresses
include memory SPD (serial presence detect 1010 000x, 1010 001x), programmable clock synthesizes
(1101 001x), clock buffers (1101 110x), thermal sensors (1001 000x), and management controllers
(vendor defined address). Contact Advantech for information on the SMBus addresses used.
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