PowerPC 405CR Embedded Controller Data Sheet
Spread Spectrum Clocking
Care must be taken when using a spread spectrum clock generator (SSCG) with the PPC405CR. This
controller uses a PLL for clock generation inside the chip. The accuracy with which the PLL follows the SSCG
is referred to as tracking skew. The PLL bandwidth and phase angle determine how much tracking skew
there is between the SSCG and the PLL for a given frequency deviation and modulation frequency. When
using an SSCG with the PPC405CR the following conditions must be met:
• The frequency deviation must not violate the minimum clock cycle time. Therefore, when operating the
PPC405CR with one or more internal clocks at their maximum supported frequency, the SSCG can only
lower the frequency.
• The maximum frequency deviation cannot exceed −3%, and the modulation frequency cannot exceed
40kHz. In some cases, on-board PPC405CR peripherals impose more stringent requirements (see
Note 1).
• Use the Peripheral Bus Clock for logic that is synchronous to the peripheral bus since this clock tracks the
modulation.
• Use the SDRAM MemClk since it also tracks the modulation.
Notes:
1. The serial port baud rates are synchronous to the modulated clock. The serial port has a tolerance of
approximately 1.5% on baud rate before framing errors begin to occur. The 1.5% tolerance assumes that the
connected device is running at precise baud rates.
2. IIC operation is unaffected.
Caution: It is up to the system designer to ensure that any SSCG used with the PPC405CR meets the
above requirements and does not adversely affect other aspects of the system.
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