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SMBus Controller Registers (D31:F3)
11-8
82801AA and 82801AB Datasheet
11.2.2
HST_CNT—Host Control Register
Register Offset:
Default Value:
02h
00h
Attribute:
Size:
R/W
8-bits
Bit
Description
7
Reserved.
6
START—WO.
This write-only bit is used to initiate the command described in the SMB_CMD field.
All registers should be setup prior to writing a ‘1’ to this bit position. This bit always reads zero. The
HOST_BUSY bit in the Host Status register (offset 00h) can be used to identify when the ICH has
finished the command.
5
LAST_BYTE—WO.
This bit is used for Block Read commands.
1 = Software sets this bit to indicate that the next byte will be the last byte to be received for the
block. The ICH will send a NOT ACK (instead of an ACK) after receiving the last byte.
4:2
SMB_CMD.
The bit encoding below indicates which command the ICH is to perform. If enabled, the
ICH will generate an interrupt or SMI# when the command has completed If the value is for a non-
supported or reserved command, the ICH will set the device error (DEV_ERR) status bit and
generate an interrupt when the START bit is set. The ICH will perform no command, and will not
operate until DEV_ERR is cleared.
000 = Quick
: The slave address and read/write value (bit 0) are stored in the transmit slave address
register.
001 = Byte
: This command uses the transmit slave address and command registers. Bit 0 of the
slave address register determines if this is a read or write command.
010 = Byte Data
: This command uses the transmit slave address, command, and DATA0 registers.
Bit 0 of the slave address register determines if this is a read or write command. If it is a read,
the DATA0 register will contain the read data.
011 = Word Data
: This command uses the transmit slave address, command, DATA0 and DATA1
registers. Bit 0 of the slave address register determines if this is a read or write command. If it
is a read, after the command completes, the DATA0 and DATA1 registers will contain the read
data.
100 = Process Call:
This command uses the transmit slave address, command, DATA0 and DATA1
registers. Bit 0 of the slave address register determines if this is a read or write command.
After the command completes, the DATA0 and DATA1 registers will contain the read data.
101 = Block
: This command uses the transmit slave address, command, DATA0 registers, and the
Block Data Byte register. For block write, the count is stored in the DATA0 register and
indicates how many bytes of data will be transferred. For block reads, the count is received
and stored in the DATA0 register. Bit 0 of the slave address register selects if this is a read or
write command. For writes, data is retrieved from the first n (where n is equal to the specified
count) addresses of the SRAM array. For reads, the data is stored in the Block Data Byte
register.
110 = I
2
C Read
: This command uses the transmit slave address, command, DATA0, DATA1
registers, and the Block Data Byte register. The read data is stored in the Block Data Byte
register. The ICH will continue reading data until the NAK is received.
111
= Reserved
1
KILL.
1 = When set, kills the current host transaction taking place, sets the FAILED status bit, and asserts
the interrupt (or SMI#) selected by the SMB_INTRSEL field.
0 = This bit, once set, must be cleared to allow the SMBus Host Controller to function normally.
0
INTREN.
Enable the generation of an interrupt or SMI# upon the completion of the command.