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Table 2-2
82371AB Pin Descriptions
Name
Type
PCI BUS INTERFACE
AD[31:0]
I/O
C/BE#[3:0]
I/O
CLKRUN#
I/O
DEVSEL#
I/O
FRAME#
I/O
IDSEL
I
Major Chips Description
PCI ADDRESS/DATA. AD[31:0] is a multiplexed address and data bus. During
the first clock of a transaction, AD[31:0] contain a physical byte address (32 bits).
During subsequent clocks, AD[31:0] contain data. A PIIX4 Bus transaction
consists of an address phase followed by one or more data phases. Little-endian
byte ordering is used. AD[7:0] define the least significant byte (LSB) and
AD[31:24] the most significant byte (MSB). When PIIX4 is a Target, AD[31:0] are
inputs during the address phase of a transaction. During the following data
phase(s), PIIX4 may be asked to supply data on AD[31:0] for a PCI read, or
accept data for a PCI write. As an Initiator, PIIX4 drives a valid address on
AD[31:2] and 0 on AD[1:0] during the address phase, and drives write or latches
read data on AD[31:0] during the data phase.
During Reset: High-Z After Reset: High-Z During POS: High-Z
BUS COMMAND AND BYTE ENABLES. The command and byte enable signals
are multiplexed on the same PCI pins. During the address phase of a transaction,
C/BE[3:0]# define the bus command. During the data phase C/BE[3:0]# are used
as Byte Enables. The Byte Enables determine which byte lanes carry meaningful
data. C/BE0# applies to byte 0, C/BE1# to byte 1, etc. PIIX4 drives C/BE[3:0]#
as an Initiator and monitors C/BE[3:0]# as a Target.
During Reset: High-Z After Reset: High-Z During POS: High-Z
CLOCK RUN#. This signal is used to communicate to PCI peripherals that the
PCI clock will be stopped. Peripherals can assert CLKRUN# to request that the
PCI clock be restarted or to keep it from stopping. This function follows the
protocol described in the PCI Mobile Design Guide, Revision 1.0.
During Reset: Low After Reset: Low During POS: High
DEVICE SELECT. PIIX4 asserts DEVSEL# to claim a PCI transaction through
positive decoding or subtractive decoding (if enabled). As an output, PIIX4
asserts DEVSEL# when it samples IDSEL active in configuration cycles to PIIX4
configuration registers. PIIX4 also asserts DEVSEL# when an internal PIIX4
address is decoded or when PIIX4 subtractively or positively decodes a cycle for
the ISA/EIO bus or IDE device. As an input, DEVSEL# indicates the response to
a PIIX4 initiated transaction and is also sampled when deciding whether to
subtractively decode the cycle. DEVSEL# is tri-stated from the leading edge of
PCIRST#. DEVSEL# remains tri-stated until driven by PIIX4 as a target.
During Reset: High-Z After Reset: High-Z During POS: High-Z
CYCLE FRAME. FRAME# is driven by the current Initiator to indicate the
beginning and duration of an access. While FRAME# is asserted data transfers
continue. When FRAME# is negated the transaction is in the final data phase.
FRAME# is an input to PIIX4 when it is the Target. FRAME# is an output when
PIIX4 is the initiator. FRAME# remains tri-stated until driven by PIIX4 as an
Initiator.
During Reset: High-Z After Reset: High-Z During POS: High-Z
INITIALIZATION DEVICE SELECT. IDSEL is used as a chip select during PCI
configuration read and write cycles. PIIX4 samples IDSEL during the address
phase of a transaction. If IDSEL is sampled active, and the bus command is a
configuration read or write, PIIX4 responds by asserting DEVSEL# on the next
cycle.
Description
2-9
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