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4–12 Functional Description
System Clocks
4.5 System Clocks
Figure 4–6 shows the AlphaPC 164SX clock generation and distribution scheme.
The AlphaPC 164SX system includes input clocks to the microprocessor as well as
clock distribution for the various system memory and I/O devices. There are other
miscellaneous clocks for ISA bus support. System clocking can be divided into the
following three main areas:
• Microprocessor input clock — The input clock runs at the operating frequency
of the 21164PC microprocessor. The AlphaPC 164SX supports cycle times from
2.50 ns to 1.88 ns. This implies input clock frequencies from 400 MHz to
533 MHz. The clock is provided by using a synthesizer. The synthesizer’s output
is used as the input clock for the 21164PC.
• Clock distribution — Clock distribution includes the distribution of system
clocks from the 21164PC microprocessor to the system logic. The AlphaPC
164SX clock distribution scheme is flexible enough to allow the majority of cycle-
time combinations to be supported. Because the PCI is synchronous to the system
clock generated by the 21164PC microprocessor, the PCI cycle time is a multiple
of the 21164PC cycle time. This distribution scheme supports a PCI operation of
33 MHz.
• Miscellaneous clocks — The miscellaneous clocks include those needed for
ISA and the combination controller. These clocks are provided by a crystal and a
frequency generator with fixed scaling.