User's Manual
Intel
®
820 Chipset Design Guide 2-65
Layout/Routing Guidelines
2.15 USB
The following are general guidelines for the USB interface:
•
Unused USB ports should be terminated with 15 KΩ pulldown resistors on both P+/P- data
lines.
•
15 Ω series resistors should be placed as close as possible to the ICH (<1 inch). These series
resistors are required for source termination of the reflected signal.
•
47 pF caps must be placed as close to the ICH as possible and on the ICH side of the series
resistors on the USB data lines (P0+/-, P1+/-). These caps are there for signal quality (rise/fall
time) and to help minimize EMI radiation.
•
15 KΩ ±5% pulldown resistors should be placed on the USB side of the series resistors on the
USB data lines (P0+/-, P1+/-), and are REQUIRED for signal termination by USB
specification. The length of the stub should be as short as possible.
•
The trace impedance for the P0+/-, P1+/- signals should be 45 Ω (to ground) for each USB
signal P+ or P-. Using the stackup recommended in section Section 5.3, “Stackup
Requirement” on page 5-1. USB requires 9 mils traces. The impedance is 90 Ω between the
differential signal pairs P+ and P- to match the 90 Ω USB twisted pair cable impedance. Note
that twisted pair characteristic impedance of 90 Ω is the series impedance of both wires,
resulting in an individual wire presenting a 45 Ω impedance. The trace impedance can be
controlled by carefully selecting the trace width, trace distance from power or ground planes,
and physical proximity of nearby traces.
•
USB data lines must be routed as critical signals. The P+/P- signal pair must be routed together
and not parallel with other signal traces to minimize crosstalk. Doubling the space from the
P+/P- signal pair to adjacent signal traces will help to prevent crosstalk. Do not worry about
crosstalk between the two P+/P- signal traces. The P+/P- signal traces must also be the same
length. This will minimize the effect of common mode current on EMI.
Figure 2-54 illustrates the recommended USB schematic.
Figure 2-54. USB Data Signals
15k
15k
15 ohm
15 ohm
47 pf
47 pf
ICH
P+
P-
USB Connector
< 1"
< 1"
90 ohm
45 ohm
45 ohm
Driver
Driver
USB Twisted Pair CableTransmission Line
Motherboard Trace
Motherboard Trace