AN2536 Freescale Semiconductor / Motorola, AN2536 Datasheet - Page 7

• Manufacturer Part Number: AN2536
• Description: MC9328MX1 and MC9328MXL High Speed Layout Design Guidelines
• Manufacturer: Freescale Semiconductor / Motorola

For differential trace, proper routing techniques are important for high-speed design:
Power Noise Filtering
To decrease the low-frequency (< 1 kHz) noise caused by the power supply, filter the noise on power lines
at the point where the power connects to the PCB and to each device. Place a 100-uF electrolytic capacitor
where the power supply lines enter the PCB. If you use a voltage regulator, place the capacitor
immediately after the pin that provides the VCC signal to the device(s). (Capacitors not only filter low-
frequency noise from the power supply, but also supply extra current when many outputs switch
simultaneously in a circuit.)
To filter power supply noise, use a non-resonant, surface-mount ferrite bead large enough to handle the
current in series with the power supply. Place a 10- to 100-uF bypass capacitor next to the ferrite bead. (If
proper termination, layout, and filtering eliminate enough noise, you do not need to use a ferrite bead.) The
ferrite bead acts as a short for high frequency noise coming from the VCC source. Any low frequency
MOTOROLA
•
•
•
•
•
•
•
•
Place a ground plane next to the outer layer to minimize noise. If you use an inner layer to route the
clock trace, sandwich the layer between reference planes.
Terminate clock signals to minimize reflection.
Use point-to-point clock traces as much as possible.
Make sure D > 2S to minimize the crosstalk between the two differential pairs.
To minimize reflection noise, place the differential traces S = 3H as they leave the device.
Keep the distance between the differential traces (S) constant over the entire trace length.
Keep the length of the two differential traces the same to minimize the skew and phase difference.
Avoid using multiple via, because they can cause impedance mismatch and inductance.
Figure 9. Height vs. Dielectric Constant Using Equation 3 and 5,
Figure 10. Example of Separating Traces for Crosstalk
keeping impedance, width, and thickness constant
Freescale Semiconductor, Inc.
For More Information On This Product,
MC9328MX1/MXL Application Note
Go to: www.freescale.com
Design Consideration
7