The extreme case of this phenomenon is when you disconnect the antenna from an RF transmitter. If you already have an intrinsically high voltage on the bus, the superimposed (and higher power) transients may not be tolerable by the device. On another note, reflections or signaling imperfections will result in over/undershoots. Current mode driving (which ensures the speed) used in many modern buses, USB included, allows lower voltage swings on the data lines. If you think of the same issue in the other direction, a higher voltage level will be harder to achieve from the driver perspective thus will limit transmission speed. In particular, power consumption is amplified when you have very high speed signals, or multiple load points. The higher the voltage/bit rate is, the higher the power consumption is (this should be obvious to the reader). It is by all means more complicated than the previous, especially on bidirectional bus like the USB.Ī primary factor when determining voltage levels for a differential bus is power consumption.

It requires a level shifter (built-in or external) and both supply voltages. Although an input can be easily made to be 5V-tolerant, on an output it is not possible to output 5V using single supply voltage. In contrast, if the data bus would be choosen to operate on 5V levels, it would cause problems for 3.3V devices. The decades old TTL standard already required only 2.4V as high level, so in theory are 3.3V compatible (as an input).

An 5V device's input/output can be designed to interpret and output 3.3V max. The case for data pins is also for supporting both 3.3V and 5V devices as simple as possible. The latter would require a switch mode boost converter which is more complex. In case of a 3.3V low power device, it is far more simpler, cheaper and more efficient to regulate 3.3V from 5V using a simple LDO than vica versa.Using 3.3V as supply voltage and increasing the current would not be equally good, as it would require thicker wire to transmit. external HDD) using higher voltage at the same supply current yields more power. For the devices that needs higher power (eg.There are (at least) two advantages of using 5V as power supply voltage instead of 3.3V: At the time USB was introduced both 5V and 3.3V devices were common and the goal was to support both systems. The 5V voltage on power pins is just a power feed for a device which needs power.

Usb vs usb 2 vs usb 3 voltage