Objective Development Forums 2010-12-20T02:18:21+02:00 https://forums.obdev.at/app.php/feed/topic/5284 2010-12-20T02:18:21+02:00 2010-12-20T02:18:21+02:00 https://forums.obdev.at/viewtopic.php?t=5284&p=17076#p17076 <![CDATA[Re: Why are the 68ohm resistor 68ohm?]]> 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 :D

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.

Statistics: Posted by Mike J. — Mon Dec 20, 2010 2:18 am

]]> 2010-12-19T19:44:19+02:00 2010-12-19T19:44:19+02:00 https://forums.obdev.at/viewtopic.php?t=5284&p=17072#p17072 <![CDATA[Why are the 68ohm resistor 68ohm?]]>
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?

Statistics: Posted by Guest — Sun Dec 19, 2010 7:44 pm

]]>