W167B Datasheet, PDF (4/18 Pages) Cypress Semiconductor – 133-MHz Spread Spectrum FTG for Pentium II Platforms

PRELIMINARY
W167B
CPU/PCI Frequency Selection
CPU frequency is selected with I/O pins 7, 8, 27, (SEL2/PCI0,
SEL1/PCI1, 48MHz/SEL0, respectively) and input pin 25
(SEL133/100#). Refer to Table 1 for CPU/PCI frequency pro-
gramming information. Additional frequency selections are
available through the serial data interface. Refer to Table 5 on
page 9.
Output Buffer Configuration
Clock Outputs
All clock outputs are designed to drive serial terminated clock
lines. The W167B outputs are CMOS-type which provide rail-
to-rail output swing.
Crystal Oscillator
The W167B requires one input reference clock to synthesize
all output frequencies. The reference clock can be either an
externally generated clock signal or the clock generated by the
internal crystal oscillator. When using an external clock signal,
pin X1 is used as the clock input and pin X2 is left open.
The internal crystal oscillator is used in conjunction with a
quartz crystal connected to device pins X1 and X2. This forms
a parallel resonant crystal oscillator circuit. The W167B incor-
porates the necessary feedback resistor and crystal load ca-
pacitors. Including typical stray circuit capacitance, the total
load presented to the crystal is approximately 18 pF. For opti-
mum frequency accuracy without the addition of external ca-
pacitors, a parallel-resonant mode crystal specifying a load of
18 pF should be used. This will typically yield reference fre-
quency accuracies within ±100 ppm.
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