The voltage levels for a USB differential signal are based on the voltage difference between the D+ and D- data lines.
USB (Universal Serial Bus) utilizes differential signaling for data transmission on the D+ and D- lines. This method transmits data by measuring the difference in voltage between the two lines, rather than referencing a single signal to ground. This approach offers better noise immunity, especially over longer cables and at higher speeds.
The specific voltage levels define whether a '1' or a '0' is being transmitted differentially. According to the provided reference, these levels are:
USB Differential Signal Voltage Levels
| Signal Type | D+ Voltage | D- Voltage |
|---|---|---|
| Differential '1' | Greater than 2.8V | Less than 0.3V |
| Differential '0' | Less than 0.3V | Greater than 2.8V |
This table clearly illustrates the high and low levels for each line relative to the other line, defining the differential state.
Key Considerations
- Differential Nature: Data is interpreted based on the difference (D+ minus D-) rather than the absolute voltage on a single line.
- Specific Thresholds: A differential '1' occurs when D+ is significantly higher than D-, specifically D+ > 2.8V and D- < 0.3V. Conversely, a differential '0' occurs when D- is significantly higher than D+, specifically D- > 2.8V and D+ < 0.3V.
- Pull Resistors: As noted in the reference, the signaling involves specific pull-up and pull-down resistors to establish quiescent states and help define the signal levels. A differential '1' involves pulling D+ over 2.8V using a 15K ohm resistor pulled to ground and pulling D- under 0.3V with a 1.5K ohm resistor pulled to 3.6V. For a differential '0', D- is pulled greater than 2.8V and D+ less than 0.3V with the same resistor configurations. These resistors are crucial for defining the default bus states (like Idle) and facilitating speed identification (e.g., Full-Speed vs. Low-Speed).
Understanding these precise voltage thresholds is fundamental to designing and troubleshooting USB interfaces, ensuring reliable communication between devices.
