........................Document #: 38-07509 Rev. *B Page 7 of 19
Special Functions
PCIF and IOAPIC Clock Outputs
The PCIF clock outputs are intended to be used, if required,
for systems IOAPIC clock functionality. ANY two of the PCIF
clock outputs can be used as IOAPIC 33-MHz clock outputs.
They are 3.3V outputs will be divided down via a simple
resistive voltage divider to meet specific system IOAPIC clock
voltage requirements. In the event these clocks are not
required, then these clocks can be used as general PCI clocks
or disabled via the assertion of the PCI_STP# pin.
3V66_1/VCH Clock Output
The 3V66_1/VCH pin has a dual functionality that is selectable
via SMBus.
Configured as DRCG (66M), SMBus Byte0, Bit 5 = ‘0’
The default condition for this pin is to power up in a 66M
operation. In 66M operation this output is SSCG capable and
when spreading is turned on, this clock will be modulated.
Configured as VCH (48M), SMBus Byte0, Bit 5 = ‘1’
In this mode, the output is configured as a 48-MHz non-spread
spectrum output. This output is phase aligned with the other
48M outputs (USB and DOT), to within 1 ns pin-to-pin skew.
The switching of 3V66_1/VCH into VCH mode occurs at
system power on. When the SMBus Bit 5 of Byte 0 is
programmed from a ‘0’ to a ‘1’, the 3V66_1/VCH output may
glitch while transitioning to 48M output mode.
PD# (Power-down) Clarification
The PD# (Power-down) pin is used to shut off ALL clocks prior
to shutting off power to the device. PD# is an asynchronous
active LOW input. This signal is synchronized internally to the
device powering down the clock synthesizer. PD# is an
asynchronous function for powering up the system. When PD#
is low, all clocks are driven to a LOW value and held there and
the VCO and PLLs are also powered down. All clocks are shut
down in a synchronous manner so has not to cause glitches
while transitioning to the low ‘stopped’ state.
PD#—Assertion
When PD# is sampled LOW by two consecutive rising edges
of the CPUC clock, then on the next HIGH-to-LOW transition
of PCIF, the PCIF clock is stopped LOW. On the next
HIGH-to-LOW transition of 66Buff, the 66Buff clock is stopped
LOW. From this time, each clock will stop LOW on its next
HIGH-to-LOW transition, except the CPUT clock. The CPU
clocks are held with the CPUT clock pin driven HIGH with a
value of 2 x Iref, and CPUC undriven. After the last clock has
stopped, the rest of the generator will be shut down.
Table 4. Spread Spectrum
SS2 SS1 SS0 Spread Mode Spread%
0 0 0 Down +0.00, –0.25
0 0 1 Down +0.00, –0.50
0 1 0 Down +0.00, –0.75
0 1 1 Down +0.00, –1.00
1 0 0 Center +0.13, –0.13
1 0 1 Center +0.25, –0.25
1 1 0 Center +0.37, –0.37
1 1 1 Center +0.50, –1.50
66Buff
PCIF
PWRDWN#
CPU 133MHz
CPU# 133MHz
3V66
66In
REF 14.318MHz
USB 48MHz
Figure 1. Power-down Assertion Timing Waveforms—Buffered Mode
