I am thinking about adding USB support to a device of mine using V-USB. From what I read there and on other sites USB seems to have only 3.3V as a high level on the data pins, whereas the voltage supplied by USB is 5V.
What is the reason behind that? To me it seems to only make things more complicated since that way I need to work with multiple voltages on the board or completely step down the Vcc to 3.3V.
The higher voltage allows compensation for voltage drop to the device. If USB was 3.3v then if you had a long cable and poor connectors with 0.5v of drop then the device will only run at 2.8v. If the voltage is 5v the you still have 4.5v to work with and that is enough to run an LDO voltage regulator.
The data lines on low speed USB have a differential signal voltage of the following characteristic for the transmitter: -
On low and full speed devices, a differential ‘1’ is transmitted by pulling D+ over 2.8V with a 15K ohm resistor pulled to ground and D- under 0.3V with a 1.5K ohm resistor pulled to 3.6V. A differential ‘0’ on the other hand is a D- greater than 2.8V and a D+ less than 0.3V with the same appropriate pull down/up resistors.
And for the receiver the spec is: -
The receiver defines a differential ‘1’ as D+ 200mV greater than D- and a differential ‘0’ as D+ 200mV less than D-.
Information taken from here and note that where it says 3V6 it actually means 3V3.
For high speed USB systems the voltage levels are smaller: -
As you can probably tell the transmit logic levels have nothing really to do with either 5V or 3V3 logic systems. The power feed is just a regular power feed that makes compatibility with 5V and 3V3 systems fairly easy.
The 5V voltage on power pins is just a power feed for a device which needs power. At the time USB was introduced both 5V and 3.3V devices were common and the goal was to support both systems. There are (at least) two advantages of using 5V as power supply voltage instead of 3.3V:
The case for data pins is also for supporting both 3.3V and 5V devices as simple as possible. An 5V device's input/output can be designed to interpret and output 3.3V max. as high level. The decades old TTL standard already required only 2.4V as high level, so in theory are 3.3V compatible (as an input).
In contrast, if the data bus would be choosen to operate on 5V levels, it would cause problems for 3.3V devices. 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. It requires a level shifter (built-in or external) and both supply voltages. It is by all means more complicated than the previous, especially on bidirectional bus like the USB.
A primary factor when determining voltage levels for a differential bus is power consumption. The higher the voltage/bit rate is, the higher the power consumption is (this should be obvious to the reader). In particular, power consumption is amplified when you have very high speed signals, or multiple load points. 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. Current mode driving (which ensures the speed) used in many modern buses, USB included, allows lower voltage swings on the data lines.
On another note, reflections or signaling imperfections will result in over/undershoots. If you already have an intrinsically high voltage on the bus, the superimposed (and higher power) transients may not be tolerable by the device. That power also goes in vain. The extreme case of this phenomenon is when you disconnect the antenna from an RF transmitter. If you have enough power in the transmitter you will jeopardize the radio. You can take other factors, like EMI, into consideration as well. How about the dissapated heat in termination? For a given Z0 more volatge, more heat.
That is why the Low/Full speed USB uses 3.3V, USB 2.0 and later uses the even lower 800/400mv. We usually want to apply the lowest voltage that makes sense for the specific interface. Be reminded that many high speed interfaces (such as ethernet, can, hdmi, pci, lvds, and many more) all use low voltage signals in the same tier.
The other reason can be confidence of connection's correctly working. Bigger range is more powerful against noise(Because needs noise with higher voltage to change bit's state).
USB seems to have only 3.3V as a high level on the data pins, whereas the voltage supplied by USB is 5V. What is the reason behind that?
Short answer is: mostly economics. USB framework was envisioned as an universal connectivity between personal computers and multitude of their of peripherals, to replace multi-pin connectors as RS-232 and Parallel Ports. This is/was a market for billions of units. Therefore, to be adopted and be successful, the new (in 1995) standard must be cheap, so re-using existing silicon technology and power distribution standards at that time would achieve this goal.
As examples from many comments and answers show, technically there is no much relationship between the level of power supply and amplitude of digital signals. The function of USB is two-fold - provide power to a peripheral, and to transfer information. These functions have somewhat different aims, and it should be of no surprise that heir voltage levels are different.
In 1995 the most developed power supply technology did have the mainstream voltage level of +5V, all primary power was based on +5V. The +5V is still the primary supply in personal computing today. So the choice of power delivery for USB peripherals as +5V was dictated by economy of scale in PC market.
Same goes for the choice +3.3V signaling amplitude, as 3.3V CMOS technology took over all older 5-V TTL (and other) drivers in consumer segment (at that time), and also provided a path for integration of USB PHY with data-processing IC. Most common I/O voltage levels on older MCU is 3.3V.
We also need to keep in mind that whoever has a leading position in development of some technology, they would dictate the selection of component base (at the time of inception of the standard). This was the era of first Intel Pentium processors.
