CY28339
Dial-a-Frequency Features
Special Functions
SMBus Dial-a-Frequency feature is available in this device via
Byte8 and Byte9.
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 that these clocks are not
required, they can be used as general PCI clocks or disabled
via the assertion of the PCI_STOP# pin.
P is a large-value PLL constant that depends on the frequency
selection achieved through the hardware selectors (S1, S0).
P value may be determined from Table 2.
Table 2. P Value
S(1:0)
0 0
P
32005333
48008000
96016000
64010667
0 1
3V66_1/VCH Clock Output
1 0
The 3V66_1/VCH pin has a dual functionality that is selectable
via SMBus.
1 1
Configured as DRCG (66M), SMBus Byte0, Bit 5 = “0”
Dial-a-dB Features
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.
SMBus Dial-a-dB feature is available in this device via Byte8
and Byte9.
Configured as VCH (48M), SMBus Byte0, Bit 5 = “1”
Spread Spectrum Clock Generation (SSCG)
In this mode, output is configured as a 48-Mhz non-spread
spectrum output that is phase-aligned with 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.
Spread Spectrum is a modulation technique used to
minimizing EMI radiation generated by repetitive digital
signals. A clock presents the greatest EMI energy at the center
frequency it is generating. Spread Spectrum distributes this
energy over a specific and controlled frequency bandwidth
therefore causing the average energy at any one point in this
band to decrease in value. This technique is achieved by
modulating the clock away from its resting frequency by a
certain percentage (which also determines the amount of EMI
reduction). In this device, Spread Spectrum is enabled by
setting specific register bits in the SMBus control bytes.
Table 3 is a listing of the modes and percentages of Spread
Spectrum modulation that this device incorporates.
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.
Table 3. Spread Spectrum
SS2 SS1
SS0
0
Spread Mode
Down
Spread%
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
+0.00, –0.25
+0.00, –0.50
+0.00, –0.75
+0.00, –1.00
+0.13, –0.13
+0.25, –0.25
+0.37, –0.37
+0.50, –1.50
1
Down
PD# Assertion
0
Down
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 × Iref, and CPUC undriven. After the last clock has
stopped, the rest of the generator will be shut down.
1
Down
0
Center
Center
Center
Center
1
0
1
3V66-0
Tpci
PCI
PCI_F
Figure 1. Unbuffered Mode – 3V66_0 to PCI and PCIF Phase Relationship
Rev 1.0,November 25, 2006
Page 6 of 17
SPECTRALINEAR [ SPECTRALINEAR INC ]
