AN2638 Freescale Semiconductor / Motorola, AN2638 Datasheet - Page 7

• Manufacturer Part Number: AN2638
• Description: Effects of Clock Jitter on the MPC8260 (HiP3 and HiP4)
• Manufacturer: Freescale Semiconductor / Motorola

3.3 XFC Filter
The XFC (external filter capacitor), shown in Figure 9, is critical to the operation of the main internal PLL
circuits. The value of this capacitor determines the stability of the PLL and its ability to lock quickly onto
the input signal edge and remain locked. The XFC provides control voltage for the system PLL VCO. There
are current sources within the PLL that charges the XFC. Lower capacitance responds more quickly to the
charge current; thus the PLL reacts faster but it may have overshoots/undershoots. To calculate the XFC
value, use the formula provided in Chapter 10, “Clocks,” in the MPC8260 PowerQUICC II™ Family
Reference Manual.
3.4 PLL Supply Filtering
On the MPC8260, VCCSYN/VCCSYN1 provide power to the PLL circuitry. To ensure stability of the PLL
and minimize noise level, these PLL power supply pins should be filtered with capacitors with low and high
frequency characteristics. Figure 10 shows an example of PLL supply filter circuit. Note that capacitors
should have low effective series inductance (ESL). It is recommended that each PLL supply have a
dedicated filter. For more information, refer to MPC8260 PowerQUICC II™ Design Checklist
(AN2290/D).
MOTOROLA
VDD
VDD
Vccsyn
XFC
10-Ohm
10-Ohm
Effects of Clock Jitter on the MPC8260 (HiP3 and HiP4)
Freescale Semiconductor, Inc.
For More Information On This Product,
10uF
10uF
Figure 10. PLL Supply Filter
Figure 9. XFC Filter Circuit
Go to: www.freescale.com
GNDSYN
GNDSYN
XFC- Capacitor
recommended value
0.1 uF
0.1 uF
Phase Lock Loop (PLL)
VCCSYN1
VCCSYN
7