5 Volts on USB bus ?

[Synthex]

Hello,

On AVR-CDC site "http://www.recursion.jp/avrcdc/", they add two diodes to operating the AVR at 3.6 Volts.
"The Vcc should be lower than 3.6V to avoid SYNC errors."

On this site : "http://www.obdev.at/products/avrusb/index.html" :
"D1 and D2 are a low cost relpacement for a low drop 3.3 V regulator chip. Operating the AVR at higher voltages exceeds the common mode range of many USB chips. If you need to run the AVR at 5 V, add 3.6 V zener diodes at D+ and D- to limit the voltage."

Why don't you do that on your project ?

Thanks.

[dmitri]

To be honest I don't remember how it happened:) I probably removed voltage drop diodes to have Atmega8 on AVR-CDC to operate Tx/Rx pins at the same level as Tx/Rx pins of Atmega on the digital board and then forgot to add zener diodes on D-/D+. And since it worked on my computers I just forgot about it. I'll change avr-cdc schematics later to comply with USB specs.

[Synthex]

On Atmel datasheet, i can see :

Operating Voltages :
2.7 - 5.5V (ATmega8L)
4.5 - 5.5V (ATmega8)
Speed Grades :
0 - 8 MHz (ATmega8L)
0 - 16 MHz (ATmega8)

With 2 diodes, Vcc is about 3.6V and the quartz crystal is 12 MHz ...

ATmega8 or ATmega8L ...
Is it work ?

[dmitri]

I used Atmega8L for avr-cdc. 12Mhz for it is above its specs but it works. I bought Atmega8L by mistake. If I did it now I would use Atmega8. I don't know how Atmega8/Atmega8L will handle 12MHz at 3.6V.

[Synthex]

I've built your USB board, but it doesn't work. I use Atmega8.
My computer report an "unknow device" ...

Any idea ?

Thanks

[dmitri]

Did you use avr-cdc firmware from my Download page? Does it say something like "usb device is malfunctioning"? Is this "unknown device" shown in the Windows Device Manager?

[Synthex]

Yes, i use AVR-CDC from your download page.
Windows say something like "usb device is malfunctioning" and this "unknown device" is shown in the Windows Device Manager.

Any idea ?

[dmitri]

I would suggest the following:

1. Check for shortcuts between D-/D+ and other wires/traces.
2. Check USB wiring, i.e. both D- and D+ from Atmega8 reach USB socket and not swapped around.
3. Check firmware. Read firmware from Atmega and compare to the original firmware from the Download page.
4. Try another 12MHz crystal.
5. Try another Atmega8.

[Synthex]

I check for shortcuts all wires/traces.
I check USB wiring.
I check firmware. Atmega8 is correctly programmed, fuses too.
I try two other 12MHz crystal.
I try three other Atmega8.

But nothing ... no USB.

The Atmega32 works well, MIDI works well, inputs works,
... but no USB ...

I think i will mount a 18F2550 ...

[dmitri]

Can you try another USB port/hub? Another computer?

Try to put voltage drop diodes. Try one first and then two.
If still not working, with two voltage drop diodes in place try an Atmega8 firmware from a different project(not avr-cdc) at http://www.obdev.at/products/avrusb/ . E.g. any HID type firmware must work and detected by windows as joystick/keyboard/mouse.