I tend to lose track of all my devices that can connect to my computer via USB, and I only expect the number of devices to increase, especially as so many products become embedded. These products will need one or more USB connectors to interface with a computer or other device. Whether you are using a standard USB-A or USB-B connector, or you want to use a Mini/Micro form factor, you’ll need to place a connector on your board and route it correctly.
This is rather easy to do in most layouts as the USB signalling standards are not too difficult to work with. Let’s look at what you need to do to incorporate a standard USB-A or USB-B into your PCB. You can then apply the same USB connector layout procedures to other USB form factors, such as USB Micro and USB Mini.
USB Connector Layout Guidelines: USB-A and USB-B
The critical layout rule that makes USB work so well is its use of differential pairs for routing. If you’re routing differential pairs in your schematic, you should make sure to indicate them with “p” (positive side) and “n” (negative side) on net names. This ensures your constraint manager recognizes these lines as differential pairs in your layout. In total, a USB-A or USB-B connector contains four connections: the differential pair mentioned above, a power connection, and a ground connection (see the image below).
USB connector layout and pinout diagram
The connector itself is shielded, which provides protection against interference. When taken alongside the use of differential pair routing, USB interfaces have strong resistance to interference within a board and from external sources.
The connector also needs to mount to the PCB using some small mounting holes. You should check your component datasheets for applicable hole sizes. Many USB connectors recommend using the standard 1.57 mm board thickness, so you should not place USB connectors on much thinner boards. The mounting connector crimps onto the backside of the board, so no external mounting screws are required.
When creating a USB connector layout, you won’t need a ground plane directly beneath the connector, although it is a good idea to place a ground plane beneath the differential pairs, i.e., in a microstrip configuration. Whether you need an internal ground plane below your surface layer also depends on the type of interface you are using. This is discussed more below.
USB PHY Interface
What about getting data from your MCU, MPU, or FPGA to your USB connector and to an external device? Unless your processor contains a built-in USB PHY interface, you’ll need to use a bridge component to generate data to send over a USB connector. A USB bridge is an IC that receives data using some common signalling protocols (e.g., SPI, I2C, or UART), and converts the data to the non-return to zero (NRZ) signalling used in USB. Similarly, if you are building a board to receive data over USB, you’ll need a bridge component to convert that data to one of these other signalling standards.
There are a number of components you can quickly bring into your board as a USB bridge:
- FT232RL: This SMT component is a UART to USB 2.0 bridge that will interface.
- FT201XQ-T: This QFP component is similar to the FT232RL, although it interfaces with I2C.
- TUSB7320IRKMR: This SMT component is more powerful and can support USB 3.0 (see below). This particular component targets applications like notebook computers. It can interface using a number of protocols, including PCIe.
- TUSB2077A: This component provides support up to 7 USB ports.
All these components are readily available from a variety of distributors.
USB 2.0 vs. USB 3.0
The first difference between USB 2.0 and 3.0 is the data transfer rate. USB 2.0 is rated up to 480 Mbit/s, while USB 3.0 reaches up to 4800 Mbit/s. If you are working with an application that requires extremely synchronous high data transfer rates, then go with USB 3.0, although USB 2.0 is still sufficient for many applications involving streaming audio, file transfer, and plenty of other applications. Note that USB 3.0 is backward compatible with USB 2.0.
At the board level, USB 2.0 and 3.0 use different termination schemes, depending on the impedance of the PHY component. Series resistors are used for termination, but the size and placement of the termination resistor depends on the component as different manufacturers use different output impedance specifications.
The USB connector bus has an input impedance of ~90 Ohms, while the driver output impedance ranges from ~28 to ~44 Ohms, so the resistor you need for termination will be particular to the component. If you don’t include termination resistors, an eye diagram for a data stream will not meet the requirement in the USB specifications. Be sure to check your datasheets for termination specifications, especially if the component supports both USB 2.0 and 3.0.
These blue connectors are USB 3.0 interfaces, and the black connectors are USB 2.0 interfaces.
The USB connector layout guidelines shown here can be easily brought into your next PCB when you use Upverter®. This easy-to-use browser-based platform is ideal for designing new PCBs from start to finish. You can easily pick an existing template from a vast range of open-source hardware projects, and you can find the connector and USB bridge components you need from the Parts Database.
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