Can someone explain how this V-USB circuit works?

Tom,

The difference between a zener diode and a plane old vanilla diode is that a zener will break down and conduct current when a reverse voltage exceeds a threshold. In your diagram, if we drive DIO2 with a voltage > 3.6 volts, then the zener will turn on limiting D-/D+ to ~3.6 V.

The 68 ohm resistors will help to limit the amount of current flowing through the reversed biased diode. If we are driving the signal with 5V, then the current thought the zener will be limited to approximately 20 mA. In addition, the resistors will help slow the output signal down a bit, decreasing ringing/emi/etc.

The 2.2k DIO5 resistor probably is used to indicate a low speed usb device.

Best Regards,

Argus

Will it work with a 100 ohm resistor instead of the 68 ohm? I just bought this Attiny85 and I have some spare resistors to try it but I am not sure if 100 ohms will work the same as I don't have any 68 ohm.

It wont be ideal but should work if wiring is short, so if your doing this on a breadboard or similar then it should be ok (Functional)

@Tom

For a quick lash up there is no need to worry about the zeners (except that you probably don't need them at all.)

The usual reason for adding some kind of voltage clamping to USB data lines is to ensure that they will pass EMC testing. One of the tests is to apply high voltage discharges to the signal lines to simulate the kind of thing that will happen if you touch one with a finger while you are carrying  a static charge. In dry conditions with some kinds of carpet this can happen often. Test specs vary but typically you might expect to apply 8kV from a 100pF capacitor via a 1500ohm resistor (simulated Human Body Discharge Human-body model - Wikipedia, the free encyclopedia) . The initial current into your USB line will be about 5.3A and a typical 3.6V 400mW zener will have a dynamic resistance of about 24ohms so the voltage across the zener will rise to over 100V (in real life it might not be quite so bad because of the zener's self capacity). Zeners are no use in this application. There are lots of transient protection devices available which WILL clamp the HBD but they don't work very well without careful pcb layout - on breadboard just don't bother.

MK

@Tom

For a quick lash up there is no need to worry about the zeners (except that you probably don't need them at all.)

The usual reason for adding some kind of voltage clamping to USB data lines is to ensure that they will pass EMC testing. One of the tests is to apply high voltage discharges to the signal lines to simulate the kind of thing that will happen if you touch one with a finger while you are carrying  a static charge. In dry conditions with some kinds of carpet this can happen often. Test specs vary but typically you might expect to apply 8kV from a 100pF capacitor via a 1500ohm resistor (simulated Human Body Discharge Human-body model - Wikipedia, the free encyclopedia) . The initial current into your USB line will be about 5.3A and a typical 3.6V 400mW zener will have a dynamic resistance of about 24ohms so the voltage across the zener will rise to over 100V (in real life it might not be quite so bad because of the zener's self capacity). Zeners are no use in this application. There are lots of transient protection devices available which WILL clamp the HBD but they don't work very well without careful pcb layout - on breadboard just don't bother.

MK