42
PCI-X Layout Guidelines
6.1.1
PCI Arbitration
80331 contains two PCI Arbiters to facilitate arbitration on the primary and secondary PCI buses.
Refer to the
PCI Local Bus Specification
, Revision 2.3, for more information on arbiter
algorithms. The specification essentially states that the algorithm needs to be fair to prevent any
one device from consuming to much of the PCI bandwidth.
A typical implementation of the arbitration logic is a two-level rotating round robin configuration.
A high priority status is assigned to a master request in level one and a low-level priority status is
assigned to a master request in level two. The arbiter checks each of the REQ# lines in the first
level. When none are asserted it traverses to checking level two. Once the GNT# has been asserted
to a master, this master has the lowest priority in its level.
The arbiter also conducts bus parking by driving A/D, C/BE# and PAR lines to a known value while
the bus is idle. The arbiter typically leaves the GNT# asserted to the master that used the bus last.
6.1.2
PCI Resistor Compensation
provides the recommended resistor compensation pin termination for the PCI primary
and secondary buses. The voltage at the RCOMP pins is 0.75 V and a 1/16 W resistor rating is
acceptable.
Figure 15.
PCI RCOMP
S_RCOMP
100
Ω
P_RCOMP
100
Ω
Содержание 80331
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