Freescale Semiconductor MPC8313E PowerQUICC II Pro Family Reference Manual page 194

Reset, Clocking, and Initialization
4.4 Clocking
The following external clock sources are utilized on the MPC8313E:
• System clock (SYS_CLK_IN or PCI_CLK)
• Ethernet clock (GTX_CLK125 for eTSECn or SD_REF_CLK/SD_REF_CLK for SGMII PHY
interface)
• USB clock (USB_CLK_IN for USB PHY)
• Real-time clock (RTC_CLK)
All clock inputs can be supplied using an external canned oscillator, a clock generation chip, or some other
source that provides a standard CMOS square wave input.
The system and USB clock sources can alternatively be provided using an external crystal (on-chip crystal
oscillator provided). Separate crystal (CR) inputs/outputs are provided for this case. The crystal oscillator
dissipates 50 mW of current, so this may not be the best choice for very low power applications. When a
crystal is not used, a square wave clock input must be supplied.
If SYS_CLK_IN is used as the system clock, the SYS_CR_CLK_IN (crystal input) must be tied to quiet
ground; otherwise, SYS_CLK_IN must be tied to quiet ground. Similarly, if USB_CLK_IN is used as the
USB clock, USB_CR_CLK_IN (crystal input) must be tied to quiet ground; otherwise, USB_CLK_IN
must be tied to quiet ground.
MPC8313E PowerQUICC™ II Pro Integrated Processor Family Reference Manual, Rev. 2
4-28 Freescale Semiconductor