usbvoltage
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.
If I am not getting confused, this system of resistors is required by iPhones and other i-Devices to charge at different current levels. Otherwise an error message will pop up. If you don't have an iPhone you can quite safely remove these and replace them.
Apple stopped being as 'lax' with the charging interface and started being very picky about having the official chargers. We still doubted that there was an enumeration chip inside each charger - too expensive and complex. So there must be something else going on in those data lines. Time to sacrifice an official Apple iPhone 3Gs charger!
Taking it apart, desoldering the 4 data line resistors and measuring them on our multimeter, we found the following as shown in the schematic:
The ATMega has a wide VCC range, some run at 3.3v, some at 5v. It's pins are not tolerant of an input at a higher than VCC voltage.
The Raspberry PI is 3.3v signal only. No 5v Tolerant pins.
The USB to Uart with 3.3v signal but 5v Tolerant pins means you can connect a 5v ATMega to the 3.3v USB-to-Uart, and the Uart will be fine. The opposite is not true. Connecting a 5v Uart to a Raspberry PI or 3.3v ATMega will not end well.
If you need to connect a two way signal between a 5v ATMega and 3.3v Raspberry Pi, you would want a level translator on the ATmega TX -> RPI RX line. The other way round, from RPI TX -> ATMega RX, should be good, as the ATMega has a (VCC * 0.6) Input Voltage High threshold. In this case, 5 * 0.6 = 3v. A 3v or higher signal will be read as a logic high, which works fine.
Update: I take it you mean connect a RPI to an ATMega through a usb-to-serial adaptor connect to the RPI? Then you don't need to worry about level translation. A 5v or 3.3v USB-To-Serial adaptor will work, since the adaptor is a buffer between the two.
Best Answer
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.