W48S87-04 Datasheet, PDF (5/21 Pages) Cypress Semiconductor – Spread Spectrum 3 DIMM Desktop Clock

PRELIMINARY
W48S87-04
CPU/PCI Frequency Selection
CPU frequency is selected with I/O pins 26, 7, and 8
(48MHz/FS0, PCI_F/FS1, and PCI0/FS2, respectively). Refer
to Table 1 for CPU/PCI frequency programming information.
Additional frequency selections are available through the seri-
al data interface. Refer to Table 5 on page 10.
Output Buffer Configuration
Clock Outputs
All clock outputs are designed to drive serial terminated clock
lines. The W48S87-04 outputs are CMOS-type, which provide
rail-to-rail output swing. To accommodate the limited voltage
swing required by some processors, the output buffers of
CPU0:3 use a special VDDL2 power supply pin that can be
tied to 2.5V nominal.
Crystal Oscillator
The W48S87-04 requires one input reference clock to synthe-
size 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 input threshold voltage of pin X1 is V_DD/2.
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 W48S87-04
incorporates the necessary feedback resistor and crystal load
capacitors. Including typical stray circuit capacitance, the total
load presented to the crystal is approximately 20 pF. For opti-
mum frequency accuracy without the addition of external ca-
pacitors, a parallel-resonant mode crystal specifying a load of
20 pF should be used. This will typically yield reference fre-
quency accuracies within ±100 ppm.
Dual Supply Voltage Operation
The W48S87-04 is designed for dual power supply operation.
Supply pins VDD1, VDD2, and VDD3 are connected to a 3.3V
supply and supply power to the internal core circuit and to the
clock output buffers, except for outputs CPU0:3 and IOAPIC.
Supply pins VDDL1 and VDDL2 may be connected to either a
2.5V or 3.3V supply.
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