SLLSE09J November 2009 – July 2021 TUSB1210
PRODUCTION DATA
Item | USB General Considerations |
---|---|
1.00 | USB design requires symmetrical termination and symmetrical component placement along the DP and DM paths. |
1.01 | Place the USB host controller and major components on the unrouted board first. |
1.02 | Place the USB host controller, as close as possible to the transceiver device, that is, ULPI interface traces as short as possible. |
1.03 | Route high-speed clock and high-speed USB. Route differential pairs first. Since these signals are critical and long length traces are to be avoided, it is therefore recommended to route DP/DM before routing less critical signals on the board. A similar recommendation is true for CLK, and ULPI signals which should be routed with equalized trace length. |
1.04 | Maintain maximum possible distance between high-speed clocks/periodic signals to high speed USB differential pairs and any connector leaving the PCB (such as I/O connectors, control, and signal headers or power connectors). |
1.05 | Place the USB receptacle at the board edge. |
1.06 | Maximum TI-recommended external capacitance on DP (or DM) lines is 4 pF
|
1.07 | Place the low-capacitance ESD protections as close as possible to the USB receptacle, with no other external devices in between. |
1.08 | Common mode chokes degrade signal quality, thus they should only be used if EMI performance enhancement is absolutely necessary. |
1.09 | Place the common mode choke (if required to improve EMI performance) as close as possible to the USB receptacle (but after one or more of the ESD devices). |
USB Interface (DP, DM) | |
2.00 | Separate signal traces into similar categories and route similar signal traces together, that is, DP/DM and ULPI. |
2.01 | Route the USB receptacle ground pin to the analog ground plane of the device with multiple via connections. |
2.02 | Route the DP/DM trace pair together. |
2.03 | For HS-capable devices, route the DP/DM signals from the device to the USB receptacle with an optimum trace length of 5 cm. Maximum trace length 1-way delay of 0.5 ns (7.5 cm for 67 ps/cm in FR-3). |
2.04 | Match the DP/DM trace lengths. Maximum mismatch allowable is 150 mils (≈0.4 cm). |
2.05 | Route the DP/DM signals with 90 Ω differential impedance, and 22.5≈30-Ω common-mode impedance (objective is to have Zodd ≈ Z0 = Zdiff/2 = 45 Ω). |
2.06 | Use an impedance calculator to determine the trace width and spacing required for the specific board stack up being used. |
2.07 | Keep the maximum possible distance between DP and DM signals from the other platform clocks, power sources and digital or analog signals. |
2.08 | Do not route DP/DM signals over or under crystals, oscillators, clock synthesizers, magnetic devices, or ICs that use clocks. |
2.09 | Avoid changing the routing layer for DP/DM traces. If unavoidable, use multiple vias. |
2.10 | Minimize bends and corners on DP/DM traces. |
2.11 | When it becomes necessary to turn 90°, use two 45° turns or an arc instead of making a single 90° turn. This reduces reflections on the signal by minimizing impedance discontinuities. |
2.12 | Avoid creating stubs on the DP/DM traces as stubs cause signal reflections and affect global signal quality. |
2.13 | If stubs are unavoidable, they must be less than 200 mils (≈0.5 cm). |
2.14 | Route DP/DM signals over continuous VCC or GND planes, without interruption, avoiding crossing anti-etch (plane splits), which increase both inductance and radiation levels by introducing a greater loop area. |
2.15 | Route DP/DM signals with at least 25 mils (≈0.65 mm) away from any plane splits. |
2.16 | Follow the 20×h thumb rule by keeping traces at least 20×(height above the plane) away from the edge of the plane (VCC or GND, depending on the plane the trace is over). |
2.17 | Changing signal layers is preferable to crossing plane splits if a choice must be made. |
2.18 | If crossing a plane split is completely unavoidable, proper placement of stitching capacitors can minimize the adverse effects on EMI and signal quality performance caused by crossing the split. |
2.19 | Avoid anti-etch on the ground plane. |
ULPI Interface (ULPIDATA<7:0>, ULPICLK, ULPINXT, ULPIDIR, ULPISTP) | |
3.00 | Route ULPI 12-pin bus as a 50 Ω single-ended adapted bus. |
3.01 | Route ULPI 12-pin bus with minimum trace lengths and a strict maximum of 90 mm, to ensure timing. (Timing budget 600 ps maximum 1-way delay assuming 66 ps/cm.) |
3.02 | Route ULPI 21-pin bus equalizing paths lengths as much as possible to have equal delays. |
3.03 | Route ULPI 12-pin bus as clock signals and set a minimum spacing of 3 times the trace width (S < 3W). |
3.04 | If the 3W minimum spacing is not respected, the minimum spacing for clock signals based on EMI testing experience is 50 mils (1.27 mm). |
3.05 | Route ULPI 12-pin bus with a dedicated ground plane. |
3.06 | Place and route the ULPI monitoring buffers as close as possible from the device ULPI bus (on test boards). |
USB Clock (USBCLKIN, CLK_IN1, CLK_IN0) | |
4.00 | Route the USB clock with the minimum possible trace length. |
4.01 | Keep the maximum possible distance between the USB clock and the other platform clocks, power sources, and digital and analog signals. |
4.02 | Route the USBCLKIN, CLK_IN1 and CLK_IN0 inputs as 50 Ω single-ended signals. |
USB Power Supply (VBUS, REG3V3, REG1V5, VBAT) | |
5.00 | VBUS must be a power plane from the device VBUS ball to the USB receptacle, or if a power plan is not possible, VBUS must be as large as possible. |
5.01 | Power signals must be wide to accommodate current level. |