Phase-Lock Loop (Pll) Functional Description; Differential Reference Clocks; Pll Output Clocks; Gclk - Compaq EV68A Hardware Reference Manual

Table 7–11 21264/EV68A Reset State Machine State Descriptions (Continued)
State Name Description
DOWN2 Triggered by duration counter reaching 8205 cycles, the PLL ramps GCLK frequency
down by the first divider ratio (X
effect of halving the GCLK frequency. The duration counter is set to 4108 cycles.
DOWN3 Triggered by duration counter reaching 4108 cycles, the PLL ramps frequency down by
the second divider ratio (X
effect of reducing the frequency by a factor of 16 (of the original frequency). The inter-
nal counter is set to 264 cycles.
WAIT_INTERRUPT Triggered by duration counter reaching 264 cycles, the 21264/EV68A waits for either an
unmasked clock interrupt or unmasked device interrupt from system. The interrupts are
wired to the interrupt request and enable internal registers. When an enabled interrupt
occurs, the PLL ramps back to full frequency. Subsequent to that, the built-in self-init
(BiSI) initializes arrayed structures. The SROM is not reloaded; instead, the 21264/
EV68A begins fetching code from the SYSTEM.

7.11 Phase-Lock Loop (PLL) Functional Description

The PLL multiplies the clock frequency of a differential input reference clock and
aligns the phase of its output to that differential input clock. Thus, the 21264/EV68A
can communicate synchronously on clock boundaries with clock periods that are
defined by the system.
7.11.1

Differential Reference Clocks

A skew-controlled, ac-coupled differential clock is provided to the PLL by way of
ClkIn_x . ClkIn_x are input signals to a differential amplifier. The frequency of
ClkIn_x can range from 80 MHz to 200 MHz. ClkIn_x can be sourced by a variety of
components that include PECL fanout parts or system PLLs. ClkIn_x are also the pri-
mary clock source for the 21264/EV68A when in PLL bypass mode.
7.11.2

PLL Output Clocks

The following sections summarize the PLL output clocks.
7.11.2.1

GCLK

The PLL provides an output clock, GCLK, with a frequency that can range from 400
MHz to 1.25 GHz under full-speed conditions. GCLK is the nominal onchip clock that
is distributed to the entire 21264/EV68A chip.
7.11.2.2

Differential 21264/EV68A Clocks

The EV6Clk_x output pads provide an external test point to measure the PLL phase
alignment. They do not provide a clock source. EV6Clk_x are square-wave signals
that drive rail-to-rail continually from 0 to VDD.
7.11.2.3

Nominal Operating Frequency

Under normal operating conditions, the frequency of the PLL output clock, GCLK, is a
simple function of the Y
21264/EV68A Hardware Reference Manual
Phase-Lock Loop (PLL) Functional Description
and Z equal 2 and 4, respectively). This has the
div div
and Z equal 16 and 32, respectively). This has the
div div
divider value.
div
Initialization and Configuration
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