It may sound philosophical, but what is perceived as easy is often the most difficult thing in life. And I’m talking about wrapping a gift. It’s embarrassing to say that my 7-year old son is doing a better job at gift-wrapping than his dad who’s walked the earth for more than 3 decades.
With such wisdom, I approach USB PCB routing cautiously when USB technology starts obsoleting its conventional counterparts like the PS/2 and Serial Port on the computer. Embedded controllers start introducing USB as an alternative interface, and that’s when I had my first attempt in routing both a USB hub and device design.
Do You Need To Worry About USB PCB Routing?
When I was requested to implement the USB feature into a revised PCB, it was pretty new in the embedded controller industry. Resources on the internet were scarce, and designers had to rely on technical guides and datasheets to implement USB on the PCB.
The USB has a simple interface, with a couple of differential pairs for data, power, and ground. But it is a technology that is both user-friendly and fast. PCB designers have a relatively easier job than firmware developers, as the components used in designing a PCB hub or device is minimal.
But does that mean you could take USB PCB routing lightly?
Risk of issues with USB routing increases with bus speed.
As a PCB designer, you shouldn’t be nonchalant in your design approach. With that said, some USB designs are susceptible to issues if you made a mistake. If you’re designing a USB1.1 or USB2.0 full-speed device, the maximum rate you need to contend with is 12 Mbps, and seldom causes issues.
However, when you take on USB2.0 High Speed and above, you need to start worrying about making mistakes. The USB2.0 High Speed runs at 480Mbps, and if you landed a project that works with USB3.1, you’re dealing with a blazing speed of 10Gbps.
What Happens When You Get USB PCB Routing Wrong
The physical simplicity in USB design is an open invitation for mistakes to be done. When that happened, you’ll find the design suffers from various issues. A bus-powered USB device may function erratically if it exceeds the limit supplied by the USB hub. According to the USB standard, the maximum current supplied by a USB hub is 500 mA and that’s in the high power mode.
Mistakes in differential pair routing may also lead to issues like time difference or differential skew. The USB differential pairs are signals labeled as D+ and D-. If both signal traces have mismatched lengths, the edges of the signal will arrive at the receiver at different time. This is more pronounced in higher speed protocols.
At a frequency like 480 Mbps or 10 Gbps, USB traces can be a source of EMI interfering other signals on the PCB. Of course, the effect can be mitigated by employing best practices that deal with high-speed signals.
USB PCB Routing Tips
It doesn’t matter if you’re working on a USB1.1 or the speedy USB3.1. You’ll want to make it a habit to always follow the recommended practices in your design. Here are some tips to help you survive the complications of USB design.
1. Power Budget
Whether you’re designing a USB hub or device, it’s crucial to get your power estimate right. For a USB hub, it has to source a maximum of 500 mA, and for USB devices, the total peak current consumed must be less than 500mA if it is bus-powered.
Get the power budget right in USB design.
2. ESD Protection
There are cases where the USB transceiver chip is damaged by ESD when plugging or unplugging the device. Ensure that you are implementing ESD protection by adding specialty ESD component on the USB bus.
3. EMI Control
When dealing with high-speed USB2.0 and above, you’ll want to prevent the traces from being a source of EMI to other circuitry on the PCB. Having a ground return path adjacent to or below the differential pair traces help in preventing ground noise. Also, ensure to maintain sufficient clearance between the USB module and other circuitry on the PCB.
4. Differential Pair Routing
You’ve heard that differential pair routing requires both traces to match in length and parallel to each other. Follow the advice closely when you’re routing USB signal pairs. The USB standards require the signal traces impedance to be matched at 90 ohms, which is those of the USB cable.
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