6.2.1.1.1 Clock Source Selection Guide

Reference clock jitter is an important parameter. Jitter on the reference clock degrades both the transmit eye and receiver jitter tolerance, no matter how clean the rest of the PLL is, thereby impairing system performance. Additionally, a particularly jittery reference clock may interfere with PLL lock detection mechanism, forcing the lock detector to issue an unlock signal. A good quality, low jitter reference clock is required to achieve compliance with supported USB 3.0 standards. For example, USB 3.0 specification requires the random jitter (RJ) component of either RX or TX to be 2.42 ps (random phase jitter calculated after applying jitter transfer function [JTF]). As the PLL typically has a number of additional jitter components, the reference clock jitter must be considerably below the overall jitter budget.

6.2.1.1.2 Oscillator

If an external clock source is used, XI must be tied to the clock source and XO must be left floating.

6.2.1.1.3 Crystal

Either a 20-MHz, 25-MHz, 30-MHz, or 40-MHz crystal can be selected. A parallel, 20-pF load crystal must be used if a crystal source is used.

6.2.2.1.1 USB Connector Pins Connection

The following rules apply for differential pair signals (DP/DM, SSTXP/SSTXN, and SSRXP/SSRXN):

• Keep as short as possible
• Must be trace-length matched and parallelism must be maintained
• Minimize vias and corners
• Avoid crossing plane splits and stubs

Figure 6-3 and Figure 6-4 are for visual reference only.

Figure 6-3 USB Standard-A Connector Pin Connection

Figure 6-4 USB Standard-B Connector Pin Connection

6.2.2.1.2 Clock Connections

The TUSB1310A device supports an external oscillator source or a crystal unit. If a clock is provided to XI instead of a crystal, XO is left open. Otherwise, if a crystal is used, the connection must adhere to the following guidelines.

Because XI and XO are coupled to other leads and supplies on the PCB, it is important to keep them as short as possible and away from any switching leads. It is also recommended to minimize the capacitance between XI and XO. This can be accomplished by connecting the VSSOSC lead to the two external capacitors CL1 and CL2 and shielding them with the clean ground lines. The VSSOSC must not be connected to PCB ground.

Load capacitance (C_LOAD) of the crystal varying with the crystal vendors is the total capacitance value of the entire oscillation circuit system as seen from the crystal. It includes two external capacitors CL1 and CL2 in Figure 6-5. The trace length between the decoupling capacitors and the corresponding power pins on the TUSB1310A device must be minimized. It is also recommended that the trace length from the capacitor pad to the power or ground plane be minimized.

Figure 6-5 Typical Crystal Connections

6.2.4.1.1 High-Speed Differential Routing

1. The high-speed differential pair (USB_DM and USB_DP) is connected to a type A USB connecter.
2. The differential pair traces should be routed with 90 Ω ±15% differential impedance.
3. The high-speed signal pair should be trace length matched.
4. Max trace length mismatch between high speed USB signal pairs should be no greater than 150 mils.
5. Keep total trace length to a minimum, if routing longer than eight inches contact TI to address signal integrity concerns.
6. Route differential traces first.
7. Route the differential pairs on the top or bottom layers with the minimum amount of vias possible.
8. No termination or coupling caps are required.
9. If a common mode choke is required then place the choke as close as possible to the USB connector signal pins.
10. Likewise ESD clamps should also be placed as close as possible to the USB connector signal pins (closer than the choke).
11. For more detailed information, refer to USB 2.0 Board Design and Layout Guidelines (SPRAAR7), which describes general PCB design and layout guidelines for the USB 2.0 differential pair (DP/DM).

6.2.4.1.2 SuperSpeed Differential Routing

1. SuperSpeed consists of two differential routing pairs: a transmit pair (USB_SSTXM and USB_SSTXP) and a receive pair (USB_SSRXM and USB_SSRXP).
2. Each differential pair trace must be routed with 90 Ω ±15% differential impedance.
3. The high-speed signal pair must be trace-length matched. Maximum trace length mismatch between SuperSpeed USB signal pairs must be no greater than 5 mils. The total length for each differential pair can be no longer than eight inches, which is based on the SuperSpeed USB compliance channel specification and must be avoided if at all possible. TI recommends that the SuperSpeed differential pairs be as short as possible.
4. The transmit differential pair does not have to be the same length as the receive differential pair. Keep total trace length to a minimum. Route differential traces first. Route the differential pairs on the top or bottom layers with the minimum amount of vias possible.
5. The transmitter differential pair requires 0.1-µF coupling capacitors for proper operation. The package or case size of these capacitors must be no larger than 0402. C-packs are not allowed. The capacitors must be placed symmetrically as close as possible to the USB connector signal pins.
6. If a common mode choke is required, place the choke as close as possible to the USB connector signal pins (closer than the transmitter capacitors).
7. Likewise, ESD clamps must also be placed as close as possible to the USB connector signal pins (closer than the choke and transmitter capacitors).
8. It is permissible to swap the plus and minus on either or both of the SuperSpeed differential pairs, which may be necessary to prevent the differential traces from crossing over one another. However, it is not permissible to swap the transmitter differential pair with the receive differential pair.
9. It is recommended to use a 2010 pad for the inside pins, provided no pad is used for adjacent pins. Instead, use a pad on one of the inside pins for the next pad route the trace between the outer pins to a via. There is enough space to route a 3.78-mil trace between the outside pads while leaving 5-mil spacing between the trace and pad; it is then possible to increase the trace width to 4 mils after the breakout.
10. In Figure 6-7 the red pads are USB_SS_RXP/USB_SS_RXN and the blue pads are USB_SS_TXP/USB_SS_TXN.