What Is the USB Interface?
USB (Universal Serial Bus) is a universal serial interface used to connect devices to a computer or to each other, able to carry both data and power.
How Does It Work?
Data is carried over a two-line (D+ and D−) differential pair; this provides noise immunity. In addition, supply (VBUS, usually 5V) and ground lines power the device.
Host and Device
USB is based on a host (the main device, e.g. a computer) and device (peripheral) structure. The host manages the communication; devices are recognized automatically when connected (plug-and-play).
| Version | Typical Speed |
|---|---|
| USB 2.0 | 480 Mbps |
| USB 3.x | 5-20 Gbps |
| USB-C | Connector (various speeds) |
USB Interface Chips
To add USB to a microcontroller, either a USB-capable MCU or a USB-to-serial converter chip (e.g. CH340, FT232) is used. On high-speed lines, EMI is reduced with a common mode choke.
Where Is It Used?
Computer peripherals, phone charge/data, USB flash drives, development boards and device programming.
Enumeration
When a device is plugged into USB, the host recognizes it: the device sends a self-describing "descriptor", and the host loads the appropriate driver accordingly. This automatic process is called enumeration, and the device class (HID, storage, audio) is determined at this stage.
Common Mistakes
- Impedance: the D+/D− lines must be routed to a 90Ω differential impedance.
- VBUS current: the device's current draw limit must not be exceeded.
- ESD protection: an ESD-protection element should be placed on the connector lines.
Frequently Asked Questions
Is USB-C a protocol?+
Does USB also provide power?+
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