Sm Bus And I C; Signals Definition; I 2 C Address Allocation Consideration; Table 19 Com Express Smbus And I - Emerson COM Express Carrier Design Manual

15 SM bus and I

15.1 Signals Definition

The SMBus is primarily used as an interface to manage peripherals such as serial presence detect (SPD)
on RAM, thermal sensors, PCI/PCIe devices, smart battery, etc. The devices that can connect to the
SMBus can be located on the module and carrier.
The I 2 C is similar to the SMBus. The I
2
defined by Philips. I C 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.
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 3.3 V 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.

Table 19 COM Express SMBus and I

SMBus and I2C Bus Function
General purpose user I
LVDS display parameters
SDVO parameters
System management bus
VGA display parameters
Care must be taken to that the I2C_CK and I2C_DAT are powered by the system 3.3V while the
SMB_CK and SMB_DAT are powered by the 3.3V Suspend rail as the COM ExpressTM module Base
Specification, so make sure to avoid to connect the I2C with the SMBus together.
15.2 I 2 C Address Allocation Consideration
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 synthesizers (1101 001x), clock buffers (1101 110x), thermal sensors (1001
000x), and management controllers (vendor-defined address).
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2
C
2 C (inter-integrated circuit) bus is a two-wire serial bus originally
2
C Signal Groups
Clock
2 C I2C_CK
LVDS_I2C_CK
SDVO_CK
SMB_CK
VGA_I2C_CLK
Data
I2C_DAT
LVDS_I2C_DAT
SDVO_DATA
SMB_DAT
VGA_I2C_DAT
COM Express Carrier Type 2
Design Guide