11.1.3 Differential Pairs
This section describes the layout recommendations for all the TUSB8042 differential pairs: USB_DP_XX, USB_DM_XX, USB_SSTXP_XX, USB_SSTXM_XX, USB_SSRXP_XX, and USB_SSRXM_XX.
- Must be designed with a differential impedance of 90 Ω ±10%.
- In order to minimize cross talk, it is recommended to keep high speed signals away from each other. Each pair should be separated by at least 5 times the signal trace width. Separating with ground as depicted in the layout example will also help minimize cross talk.
- Route all differential pairs on the same layer adjacent to a solid ground plane.
- Do not route differential pairs over any plane split.
- Adding test points will cause impedance discontinuity and will therefore negative impact signal performance. If test points are used, they should be placed in series and symmetrically. They must not be placed in a manner that causes stub on the differential pair.
- Avoid 90 degree turns in trace. The use of bends in differential traces should be kept to a minimum. When bends are used, the number of left and right bends should be as equal as possible and the angle of the bend should be ≥ 135 degrees. This will minimize any length mismatch causes by the bends and therefore minimize the impact bends have on EMI.
- Minimize the trace lengths of the differential pair traces. The maximum recommended trace length for SS differential pair signals and USB 2.0 differential pair signals is eight inches. Longer trace lengths require very careful routing to assure proper signal integrity.
- Match the etch lengths of the differential pair traces (i.e. DP and DM or SSRXP and SSRXM or SSTXP and SSTXM). There should be less than 5 mils difference between a SS differential pair signal and its complement. The USB 2.0 differential pairs should not exceed 50 mils relative trace length difference.
- The etch lengths of the differential pair groups do not need to match (i.e. the length of the SSRX pair to that of the SSTX pair), but all trace lengths should be minimized.
- Minimize the use of vias in the differential pair paths as much as possible. If this is not practical, make sure that the same via type and placement are used for both signals in a pair. Any vias used should be placed as close as possible to the TUSB8042 device.
- To ease routing, the polarity of the SS differential pairs can be swapped. This means that SSTXP can be routed to SSTXM or SSRXM can be routed to SSRXP.
- To ease routing of the USB2 DP and DM pair, the polarity of these pins can be swapped. If this is done, the appropriate Px_usb2pol register, where x = 0, 1, 2, 3, or 4, must be set.
- Do not place power fuses across the differential pair traces.