PCIX I/O System Clock Generator With EMI Control Features
Cypress Semiconductor Corporation
525 Los Coches St.
Milpitas, CA 95035. Tel: 408-263-6300, Fax: 408-263-6571
http://www.cypress.com
Document#: 38-07034 Rev. **
05/02/2001
Page 3 of 14
APPROVED PRODUCT
C9531
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 of its harmonics.
In this device Spread Spectrum is enabled externally through pin 15 (SSCG#) or internally via SMBus 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 modulation disabled) unless externally forced to a low. It
may also be enabled by programming SMBus Byte 0 Bit 0 LOW (to enable SMBus control of the function) and then
programming SMBus byte 0 bit 6 low to set the feature active.
Spectrum Spreading Selection Table
Output clock
Frequency
% OF FREQUENCY SPREADING
SMBus 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%)
SMBus 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)
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.
Down Spread
Down Spread
Down Spread
Down Spread
Spread off
Spread on
Center Frequency,
Spread off
Center Frequency,
Spread on