+/+…when timing is critical
C9530
PCIX I/O System Clock Generator With EMI Control Features
Preliminary
Spectrum Spread Clocking
Down Spread Description
Spread Spectrum is a modulation technique for distributing clock period over a certain bandwidth (called Spread
Bandwidth). This technique allows the distribution of the undesirable electromagnetic energy (EMI) over a wide range
of frequencies therefore reducing the average radiated energy present at any frequency over a given time period. As
the spread is specified as a percentage of the resting (non-spread) frequency value, it is effective at the fundamental
and, to a greater extent, at all its harmonics.
In this device, Spread Spectrum is enabled externally through pin 27 (SSCG#) or internally via I2C Byte 0 Bit 0 and 6.
Spread spectrum is enabled externally when the SSCG# pin is low. This pin has an internal device pull up resistor,
which causes its state to default to a high (Spread Spectrum disabled) unless externally forced to a low. It may also be
enabled by programming I2C Byte 0 Bit 0 LOW (to enable I2C control of the function) and then programming I2C Byte 0
Bit 6 LOW to set the feature active.
Spread off
Spread on
Center Frequency,
Spread on
Center Frequency,
Spread off
Spectrum Spreading Selection Table
Output clock
Frequency
% OF FREQUENCY SPREADING
I2C Byte 0 Bit 5 =0
1.0% (-1.0% + 0%)
1.0% (-1.0% + 0%)
1.0% (-1.0% + 0%)
1.0% (-1.0% + 0%)
I2C Byte 0 Bit 5 =1
0.5% (-0.5% + 0%)
0.5% (-0.5% + 0%)
0.5% (-0.5% + 0%)
0.5% (-0.5% + 0%)
MODE
33.3 MHz (XIN)
66.6 MHz (XIN*2)
100.0 MHz (XIN*3)
133.3 MHz (XIN*4)
Down Spread
Down Spread
Down Spread
Down Spread
When SSCG is enabled, the device will down spread the clock over a range that is 1% of its resting frequency. This
means that for a 100 MHz output clock the frequency will sweep through a spectral range from 99 to 100 MHz.
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST.
MILPITAS, CA 95035 TEL: 408-263-6300, FAX: 263-6571
http://www.imicorp.com
Rev. 1.2
3/12/2000
Page 3 of 13
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