AN2638 Freescale Semiconductor / Motorola, AN2638 Datasheet - Page 8

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

How to Minimize Clock Jitter
How to Minimize Clock Jitter
4 How to Minimize Clock Jitter
The following are possible ways to reduce clock jitter and avoid AC timing violations.
4.1 Power Supply
A main factor that can contribute to clock jitter is the presence of significant voltage drops in a design’s
power supply. Therefore, it is important to ensure that both the device’s power and ground input pins have
proper routing on the printed circuit board and to maintain a stable device voltage at all times. A poorly
designed power source or board layouts may contribute to clock jitter during increased I/O activity and CPU
activity on device.
VDDL voltage should not drop below the minimum recommended operating condition as specified in the
appropriate hardware specification document (refer to Table 2).
4.2 XFC RC Filter
On systems where excessive PLL jitter causes AC timing violations, a 2.4Kohm resister (R
added in series with the specified (C
this resister/capacitor (RC) filter acts as a noise rejection circuit, thus allowing for fast clock recovery for a
wider frequency bandwidth and less susceptibility to phase noise or jitter. For the best result, the XFC
capacitor should be placed as close as possible to the XFC pin on the MPC8260. The 2.4Kohm R
stays constant for all multiplication factors. For selecting C
PowerQUICC II Family Reference Manual. Figure 11 shows the physical connection of the XFC RC filter
circuit.
5 Jitter Measurement
Before proceeding to jitter measurement, the user must verify that the system power supply can provide
clean power to the input pins on the MPC8260. It is critical that the measuring method is accurate and
repeatable. The following steps will help to ensure that repeatable timing measurements are obtained:
8
1. For clock jitter measurement we used Agilent Infinium Scope (2 channels @ 2 GSamples / sec.)
2. For best measurements connect the probe tips as close as possible to the SDRAM
with an Agilent 1160 passive probe. The probe used had a bandwidth of 500Mhz (scope + probe)
and a compensation range of (6 - 9) pF. Other manufactures also have equipment available for these
measurements.
and CAS signals.
Vccsyn
XFC
Effects of Clock Jitter on the MPC8260 (HiP3 and HiP4)
Freescale Semiconductor, Inc.
For More Information On This Product,
XFC
Figure 11. External PLL filter
Go to: www.freescale.com
) capacitor. As discussed in Section 3.1, “PLL Transfer Function,”
R
C
(refer to XFC multiplication factor)
XFC
XFC
XFC
2.4K ohm
capacitor values, refer to MPC8260
CLOCK_IN
MOTOROLA
XFC
XFC
) can be
value
pin