Specifications

4–16 Functional Description
Subject to Change – 17 January 1997
System Clocks
4.6 System Clocks
Figure 4–7 shows the AlphaPC 164 clock generation and distribution scheme.
The AlphaPC 164 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 164 supports cycle times from
2.72 ns to 2.0 ns. This implies input clock frequencies from 366 MHz to
500 MHz. The clock is provided by using a relatively low-frequency oscillator
whose output is fed into a PLL. The PLL multiplies the input frequency by a
factor of 10. This 10x frequency is then used as the 21164 input clock.
•
Clock distribution — Clock distribution includes the distribution of system
clocks from the 21164 microprocessor to the system logic. The AlphaPC 164
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 allows a range of supported PCI
clock combinations between 25 MHz and 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.
The default microprocessor input clock oscillator runs at 36.66 MHz. A TriQuint
TQ2060 PLL multiplier synthesizes a higher-frequency CPU clock (signals
clk_in_h and clk_in_l) and drives the 21164 differential clock inputs at
366.6 MHz. The 21164 microprocessor uses this clock to generate its internal
366.6-MHz clock. The divide-by-one function (normal state) is set in the 21164
microprocessor (clk_mode_h<2:0> input pins = 101). This oscillator is
socketed. Other oscillators with different frequencies can be substituted. Refer to
Section 5.3.1 for examples.