
SiS5595 PCI System I/O Chipset
Preliminary V2.0 Nov. 2, 1998 36 Silicon Integrated Systems Corporation
3.3
SMBUS FUNCTIONAL DESCRIPTIONS
The System Management Bus (SMBus) is a subset of the Phillips I
2
C protocol. It is a two-
wire interface for system to obtain chips' information, to indicate device model or part
number, and so on. It also can be applied to control devices such as system
suspend/wakeup, return device status or extend I/O for control purposes.
The SMBus host controller contains a Host Master and a Host Slave. The slave address may
contain an alias address. The host master and slave are fully independent, which conveys
that the host master can communicate with the host slave via the SMBUS protocol.
SiS5595B also supports SMBALERT# signal for slave devices to assert request. The
SMBALERT# pin is multiplexed with GPIO9 and BTI.
Access the SMBus register can be achieved by issuing an I/O write to ACPI offset 38H with
an INDEX, then followed by a I/O read or write to APCI offset 39h with Data.
The SMBus Interrupt Request can be routed to any IRQ or SMI#/SCI.
Table 3.3-1 SMBus Interrupt Table
INTERRUPT TYPE
IRQ
ASSOCIATED REGISTER
SIO.7E.7 : Data Acquisition Module and SMBus IRQ Mapping First
Enable
SIO.7E.5 : SMBus IRQ Mapping Second Enable
SIO.7E.3~0 : IRQ Routing table
APCI.14.23 : SMB_STS
APCI.18.23 : SMB_EN
APCI.30.3 : SMBSMI_STS
APCI.31.3 : SMBSMI_EN
SCI/SMI#
SMI#
3.3.1 SMBUS HOST MASTER INTERFACE
SiS5595B SMBus Host Master supports full SMBus protocol, including Quick command,
Send/Receive Byte, Read/Write Byte/Word, Read/Write Block, and Process Call. The
SiS5595B supports Read/Write Block command by an 8-byte buffer instead of 32-byte. For
block transfer size larger than 8 bytes, software manipulation is required. Once the eight
bytes block data have been transferred, the Sub-Block Request Status is employed to tell
service routine some data not processed yet. For the maximun of 32 bytes block data
transfer, four SMBus Interrupts will be raised by 5595 to complete the transfer.
When a Host transfer is initiated, the HOST_BUSY status bit will be set. The software is not
allowed to start a new command protocol till the HOST_BUSY reset to 0. The currently
HOST Master transfer cycle can be stopped by writting a ‘1’ to SMB_Kill bit. When a ‘1’ is
written to SMB_Kill bit, all status bit for host master and host slave will be reset. After
HOST_BUSY status bit becomes zero, the software is allowed to initiate a new transfer.
3.3.2 SMBUS HOST SLAVE INTERFACE
The Host has a Reserved Address in 0001000xb (x is R/W bit for command protocol) and an
Alias Address defined in SMB_Alias (SMBus Reg.13h). An idividual enable and status bit is
implemented for both address decoder. If a modified Write Word has been received by Host