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Why are the 68ohm resistor 68ohm?
Posted: Sun Dec 19, 2010 7:44 pm
by paul
Hi All.
I'm guessing the D+/D- series resistor is supposed to be an impedance matching resistor, but I've seen these mentioned as < 45ohm. If that's what this one is why is it 68ohms?
Re: Why are the 68ohm resistor 68ohm?
Posted: Mon Dec 20, 2010 2:18 am
by Mike J.
Difference between AVR Pin Voltage and Voltage on USB-Data-Line:
5V - 3.3V = 1.7V
The Current over an AVR I/O-Pin: 25mA (maximum 40mA, but this is not recommended)
R=U/I
R=1.7V / 0.025A = 68 Ohm
I don't use Z-Diodes and lower resistor values (33 Ohm) because the Supply Voltage of my AVR is 3.3V and there is no currend flow over my I/O-Pins
Don't worry about the low Voltage, I test a ATmega8 @ 12MHz and he start to working at 2.55V ! (but this is a critical minimum, 2.7V or 3.3V is better for working with USB)
The ATmega644P @ 12MHz need only 1.9V for working without errors.