Datasheet
PowerPC 970FX RISC Microprocessor
3.9 Spread Spectrum Clock Generator
3.9.1 Design Considerations
When designing with the spread spectrum clock generator (SSCG), there are several design issues that must
be considered as described in this section. SSCG creates a controlled amount of long-term jitter. For a
receiving PLL in the PowerPC 970FX to operate in this environment, it must be able to accurately track the
SSCG clock jitter.
Note: The accuracy to which the PowerPC 970FX PLL can track the SSCG clock is called the tracking skew.
The following SSCG configuration is recommended:
• Down spread mode, less than or equal to 1% of the maximum frequency
• A modulation frequency of 30 kHz
• A cubic sweep profile, also called a Hershey Kiss profile (as in a Lexmark1 profile), as described in
Figure 3-9.
In this configuration the tracking skew is less than 100 ps.
Figure 3-9. Spread Spectrum Clock Generator Modulation Profile
0%
Down-spread
frequency
change
-1%
0μs
33.3 μs
Time Increases
1. See patent 5,631,920.
Electrical and Thermal Characteristics
Page 40 of 78
Version 2.5
March 26, 2007
IBM [ IBM ]
