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3.6 System Clocks
Figure 3–6 shows the AlphaPC 164UX clock generation and distribution scheme.
The AlphaPC 164UX 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 21164 microprocessor. The AlphaPC 164UX supports cycle times from
3.33ns to 1.25 ns. This implies input clock frequencies from 300MHz to 800
MHz. The clock is provided by using a TQ2061. The TQ2061's output is used as
the input clock for the 21164.
•
Clock distribution — Clock distribution includes the distribution of system
clocks from the 21164 microprocessor to the system logic. The AlphaPC 164UX
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 21164 microprocessor, the PCI cycle time is a multiple of the
21164 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.
System Clocks
Functional Description
3–15
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