User`s guide

3.7 System Clocks

Figure 3–5 shows the EB164 clock generation and distribution scheme.

The EB164 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

three main areas:

• Microprocessor input clock

The input clock runs at twice the operating frequency of the 21164. The

EB164 supports cycle times from 3.2 ns to 5.0 ns. This implies input clock

frequencies from 400 MHz to 625 MHz. The clock is provided using a

relatively low frequency oscillator whose output is fed into a phase-locked

loop (PLL). The PLL multiplies the input frequency by a factor of 20. This

20 frequency is then used as the 21164 input clock.

• Clock distribution

Clock distribution includes the distribution of system clocks from the 21164

to the system logic. The EB164 clock distribution scheme is ﬂexible enough

to allow the majority of cycle times combinations to be supported. Because

the PCI is synchronous to the system clock generated by the 21164, 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 standard microprocessor input clock oscillator runs at 26.66 MHz. A

TriQuint TQ2061 phase-locked loop (PLL) multiplier synthesizes a higher

frequency CPU clock (signals clk_in_h,l) and drives the 21164 differential

clock inputs at 533.2 MHz. The 21164 microprocessor divides this clock by two

to generate its internal 266.6-MHz clock. The divide-by-two function is set

because the 21164’s clk_mode_h<1:0> input pins are both grounded (normal

mode). This oscillator is socketed. Other oscillators with different frequencies

can be substituted. Refer to schematic page eb164.3 for examples.

3–16 Functional Description