[SOLVED] USB data indicator on D+ D- bus from AVR

[maxxsinner]

Just about to send my Gerber files in to get my boards made and it occurred to me, it would be a perfect debug tool to have a couple of LED's to indicate data flow between the USB connection and the micro controller. Nice quick and easy diagnostic tool. After googling for a while, I can not find any where that explains how best to do it? Obviously just slapping an LED on the bus will start drawing the standard 20-30 ma through the bus which I am sure will have, putting it nicely, have a detrimental effect on the bus. This is my circuit for my USB connection -

Would it work if I was to put a low current, low voltage LED on between the zenner diodes and ground? Plenty of 0805 smd leds that only draw 2mA at around 1.8v (which it would only get around 1.4 but it would be enough). I would normally test this kind of idea but I don't want to get my boards made, plug one in and find out my mother board says no with the magic blue smoke.

 

[FvM]

USB is using pull-up and pull-down resistors at D+ and D- to signal device attachment and speed. Connecting LEDs without a buffer will cause bus failure.

 

[maxxsinner]

Thanks for the reply. So something like a N type mosfet would work as a buffer? Might end up with too many extra components to make it worth while.

 

[ZASto]

No, MOSFET will influence too much with it-s G-S capacitance.
Definitely not worthwhile.

 

[maxxsinner]

Cheers ZASto. I saw it done on a PIC proto board and thought it would be easy to implement on my AVR board too. But as you both have said, it will wreak havoc with my data lines. Pretty sure that the PIC chip has dedicated pins for the USB data. Might make all the difference.

 

[FvM]

CMOS logic gates should be an acceptable additional load for full-speed USB. For high-speed, additional series resistors should be considered.

Nevertheless I'm not sure if displaying the average bus level (that's what the LED circuit does) gives the intended information.

 

[maxxsinner]

Thanks FvM. It will just assist with diagnostics so simply, if the power LED is on and the two data lines are flashing, I can almost rule out my board being the problem, it will more than likely be the PC it's connected to that is the issue.