IBM PowerPC 405GP User Manual

7.5.1 PCI Clocks
The following clocks are related to the PCI logic:
•
The on-chip PLB clock
•
The on-chip synchronous PCI clock
•
An external asynchronous PCI clock, PCIClk
In
synchronous PCI mode, the asynchronous PCI clock input is ignored and the on-chip PCllogic
runs synchronously at the selected PLB:PCI divide ratio. In synchronous mode, the derived PCI clock
must equal SysClk.
In asynchronous PCI mode, the asynchronous PCI clock attaches to logic that interfaces with the PCI
bus. The synchronous clock must always be provided to the PCI logic, even when the asynchronous
PCI clock is used. Table 7-6 describes the relationships between the synchronous PCI clock, the
asynchronous
PCI clock, and the PLB clock.
In asynchronous PCI mode, the synchronous PCI clock must meet certain requirements. The
following equation describes the relationship that must be maintained between the asynchronous PCI
clock
and synchronous PCI clock. Select an appropriate PCI:PLB ratio to maintain the relationship:
AsyncPC/c/ock-5: SyncPC/c/ock-5: ((2 x AsyncPC/c/ock) + 1 MHz)
Table 7-6 lists supported and commonly used combinations of synchronous and asynchronous PCI
clocks. In general,
higher synchronous PCI clock frequencies provides better performance, while
lower synchronous PCI clock frequencies minimize power consumption.
Table 7-6. Example Synchronous PCI Clock Frequencies in Asynchronous Mode
Asynchronous Synchronous
Sync PCI:PLB
PCI Frequency PCI Frequency PLB Frequency
Ratio
20 MHz 33.3 MHz 133.3 MHz
1 :4
33.3 MHz
100 MHz
1:3
27.6 MHz
83.3 MHz 1:3
33.3 MHz 44.4 MHz 133.3 MHz
1 :3
33.3 MHz
133.3 MHz 1:4
50 MHz 100 MHz
1:2
41.6 MHz
83.3 MHz 1:2
40 MHz
44.4 MHz 133.3 MHz
1:3
50 MHz 100 MHz
1 :2
41.6 MHz
83.3 MHz
1:2
66.6 MHz 66.6 MHz
133.3 MHz
1 :2
100 MHz
100 MHz
1
:1
83.3 MHz
83.3 MHz 1
:1
7.5.2 PCI Adapter Applications
Because various systems run PCI expansion buses at different PCI frequencies, several PCI clock
frequencies may need to be supported when the PPC405GP is used in PCI adapters. Using the PCI
7-8
PPC405GP User's Manual
Preliminary

IBM PowerPC 405GP Specifications

General

Brand	IBM
Model	PowerPC 405GP
Category	Computer Hardware
Language	English