Datasheet 215
Functional Description
5.20 SMBus Controller (D31:F3)
The PCH provides an System Management Bus (SMBus) 2.0 host controller as well as
an SMBus Slave Interface. The host controller provides a mechanism for the processor
to initiate communications with SMBus peripherals (slaves). The PCH is also capable of
operating in a mode in which it can communicate with I
2
C compatible devices.
The PCH can perform SMBus messages with either packet error checking (PEC) enabled
or disabled. The actual PEC calculation and checking is performed in hardware by the
PCH.
The Slave Interface allows an external master to read from or write to the PCH. Write
cycles can be used to cause certain events or pass messages, and the read cycles can
be used to determine the state of various status bits. The PCH’s internal host controller
cannot access the PCH’s internal Slave Interface.
The PCH SMBus logic exists in Device 31:Function 3 configuration space, and consists
of a transmit data path, and host controller. The transmit data path provides the data
flow logic needed to implement the seven different SMBus command protocols and is
controlled by the host controller. The PCH’s SMBus controller logic is clocked by RTC
clock.
The SMBus Address Resolution Protocol (ARP) is supported by using the existing host
controller commands through software, except for the new Host Notify command
(which is actually a received message).
The programming model of the host controller is combined into two portions: a PCI
configuration portion, and a system I/O mapped portion. All static configuration, such
as the I/O base address, is done using the PCI configuration space. Real-time
programming of the Host interface is done in system I/O space.
The PCH SMBus host controller checks for parity errors as a target. If an error is
detected, the detected parity error bit in the PCI Status Register (Device 31:Function
3:Offset 06h:Bit 15) is set. If Bit 6 and Bit 8 of the PCI Command Register (Device
31:Function 3:Offset 04h) are set, an SERR# is generated and the signaled SERR# bit
in the PCI Status Register (bit 14) is set.
5.20.1 Host Controller
The SMBus host controller is used to send commands to other SMBus slave devices.
Software sets up the host controller with an address, command, and, for writes, data
and optional PEC; and then tells the controller to start. When the controller has finished
transmitting data on writes, or receiving data on reads, it generates an SMI# or
interrupt, if enabled.
The host controller supports 8 command protocols of the SMBus interface (see System
Management Bus (SMBus) Specification, Version 2.0): Quick Command, Send Byte,
Receive Byte, Write Byte/Word, Read Byte/Word, Process Call, Block Read/Write, Block
Write–Block Read Process Call, and Host Notify.
The SMBus host controller requires that the various data and command fields be setup
for the type of command to be sent. When software sets the START bit, the SMBus Host
controller performs the requested transaction, and interrupts the processor (or
generates an SMI#) when the transaction is completed. Once a START command has
been issued, the values of the “active registers” (Host Control, Host Command,
Transmit Slave Address, Data 0, Data 1) should not be changed or read until the
interrupt status message (INTR) has been set (indicating the completion of the
command). Any register values needed for computation purposes should be saved prior
to issuing of a new command, as the SMBus host controller updates all registers while
completing the new command.
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