A comprehensive technical reference for USB standards, specifications, and connector types to help developers and engineers navigate the complex USB ecosystem.
USB Standards Technical Reference
USB (Universal Serial Bus) has evolved significantly since its introduction in 1996. This technical reference provides clear specifications and comparisons across all USB generations to help developers, engineers, and IT professionals select the appropriate USB standard for their needs.
USB Generations: Marketing Names vs. Technical Specifications
The USB naming conventions have become increasingly complex over time, with marketing names often differing from the technical specifications. The following table clarifies the relationship between marketing names and technical specifications:
| Marketing Name | Also Known As | Signal (Gbps) | Signal (MiB/s) | Wires | Cable Length |
|---|---|---|---|---|---|
| USB 1.1 Full Speed | - | 0.012 Gbps | 1.5 MiB/s | 4 | 4m |
| USB 2.0 Hi-Speed | - | 0.48 Gbps | 60 MiB/s | 4 | 4m |
| SuperSpeed | USB 3.0, USB 3.1, USB 3.2 | 5 Gbps | 625 MiB/s | 8 | 3m |
| SuperSpeedPlus | USB 3.1, USB 3.2 | 10 Gbps | 1250 MiB/s | 8 | 2m |
| SuperSpeedPlus | USB 3.2 | 20 Gbps | 2500 MiB/s | 12 | 1m |
| USB4 | USB4 Gen 2×2 | 20 Gbps | 2500 MiB/s | 12 | 0.8m |
| USB4 | USB4 Gen 3×2 | 40 Gbps | 5000 MiB/s | 12 | 0.8m |
USB Technical Details: Generation Naming Convention, Lanes, and Speed
Understanding the technical specifications requires knowledge of the generation naming convention, number of lanes, and effective speeds:
| Name | Signal (Mbps) | Signal Total (Mbps) | Encoding | Effective (Mbps) | Effective (Bps) | Real Life (Sequential Read) |
|---|---|---|---|---|---|---|
| USB 3.2 Gen 1x1 | 5,000 | 5,000 | 8b/10b | 4,000 | 500 MiB/s | 400 MiB/s |
| USB 3.2 Gen 1x2 | 5,000 | 10,000 | 8b/10b | 8,000 | 1,000 MiB/s | 800 MiB/s |
| USB 3.2 Gen 2x1 | 10,000 | 10,000 | 128b/132b | 9,696 | 1,212 MiB/s | 780 MiB/s |
| USB 3.2 Gen 2x2 | 10,000 | 20,000 | 128b/132b | 19,392 | 2,424 MiB/s | 1,600 MiB/s |
| USB 4 Gen 2x2 | 10,000 | 20,000 | 128b/132b | 19,392 | 2,424 MiB/s | 1,600 MiB/s |
| USB 4 Gen 3x2 | 20,000 | 40,000 | 128b/132b | 38,787 | 4,848 MiB/s | 2,700 MiB/s |
Note: Multi-lane systems use lane striping (on TX) and lane bonding (on RX).
USB Cable Configurations
USB cables come in different wire configurations, each supporting different capabilities:
4-Wire Configuration
- Wires: PWR, GND, D+, D-
- Configuration: 1 half-duplex lane
- Supported Standards: USB 1.1 Full Speed, USB 2.0 Hi-Speed
8-Wire Configuration
- Wires: PWR, GND, D+, D-, RX+, RX-, TX-, TX+
- Configuration: 2 lanes (one up, one down)
- Supported Standards: USB 3.0/3.1/3.2 SuperSpeed, USB 3.1/3.2 SuperSpeedPlus
12-Wire Configuration
- Wires: PWR, GND, D+, D-, RX1+, RX1-, RX2-, RX2+, TX1+, TX1-, TX2-, TX2+
- Configuration: 4 lanes (two up, two down)
- Supported Standards: USB 3.2 SuperSpeedPlus (20Gbps), USB4
Note: 1 USB lane = 1 twisted wire pair +/-
USB Connector Types
USB-A/B Connectors
- Type-A 4-wires: Standard USB-A connector supporting USB 1.1 and USB 2.0
- Type-A 8-wires: USB-A connector with additional pins for USB 3.0+ speeds
- Type-B 4-wires: Standard USB-B connector supporting USB 1.1 and USB 2.0
- Type-B 8-wires: USB-B connector with additional pins for USB 3.0+ speeds
USB-C Connectors
- 12 wires: Only the USB Type-C connector has enough pins to support multiple lanes
- CC1 and CC2: Downstream facing port (DFP) and upstream facing port (UFP) detection, also used for power negotiation and alt mode switching
- SBU1 and SBU2: Secondary bus wires for the DisplayPort AUX channel and hot plug detection (HPD)
Power Delivery Specifications
USB power delivery has evolved significantly, with increasing power capabilities:
| Specification | Max. Voltage | Max. Current | Max. Power |
|---|---|---|---|
| USB 2.0 | 5V | 500mA | 2.5W |
| USB 3.0 / USB 3.1 | 5V | 900mA | 4.5W |
| USB Battery Charging (BC) 1.2 | 5V | 1.5A | 7.5W |
| USB-C Current Mode (non-PD) | 5V | 3A | 15W |
| USB-C / Power Delivery (PD 1/2) | 20V | 5A | 100W |
| USB-C PD 3.1 (EPR) | 48V | 5A | 240W |
USB Standards Timeline
- USB 1.0 (January, 1996): Initial USB specification
- USB 1.1 (September, 1998): Improved version with bug fixes
- USB 2.0 (April, 2000): Introduced Hi-Speed (480 Mbps)
- USB 3.0 (November, 2008): Introduced SuperSpeed (5 Gbps)
- USB 3.1 (July, 2013): Introduced SuperSpeed+ (10 Gbps)
- USB 3.2 (September, 2017): Multi-lane operation, up to 20 Gbps
- USB 4.0 (August, 2019): Based on Thunderbolt 3, up to 40 Gbps
Practical Applications
Understanding these USB specifications is crucial for:
- Device Development: Selecting appropriate USB controllers and connectors for new hardware
- Performance Optimization: Ensuring data transfer speeds match the application requirements
- Power Management: Implementing appropriate power delivery solutions for various devices
- Compatibility: Ensuring backward compatibility while planning for future upgrades
- Troubleshooting: Diagnosing performance issues related to USB standards and cable quality
References
- Universal Serial Bus Revision 3.0 Specification
- Real-world USB 3.2 Gen 2 Performance
- USB 3.1 Tested: Performance
- World's First USB 3.2 Demonstration | Synopsys
- USB4.0 M.2 NVMe Enclosure Review
Originally created by Fabien Sanglard on May 5, 2022
Comments
Please log in or register to join the discussion