I'm currently trying to create my own Attiny85 usb thumb(to mimic an joystick). I understand that I have to limit the voltage on the attiny's pins that are connected to the D+/- pins from the usb. In various schematics there are some pull-up resistors used with some zener diodes(that will drop the voltage that exceeds 3.3 V).(taken from insidegadgets)
I am sure most computers these days have a voltage regulator built-in them(everyone can just plug the usb into the 220v socket...).Can then I just avoid an regulator?(as other schematics incorporate in them)
Why is there one pull-up resistor on pin2 only(pin2 from usb)? That is D- pin.
An resistor can already limit the voltage.Why need zener diodes?(eg.: attiny85 draws ~7mA at 16.5 MH and 5v. And we need a resistance that would drop the 5v from one pin-considered in HIGH state- to 3.3v...that is a 1.2v drop.So R=1.2/0.007=171 ohms.That resistor attached to the attiny would produce a 1.2 drop).
There are two zener diodes attached to D+/- pins. What's with the resistors?(R2/R1) They limit..the current(didn't know the pc is so sensitive..)?