Objective Development Forums

This is a strange problem but I'm determined to find a solution. I have a few V-USB-based designs, two based on ATTiny25 without a crystal ("No 1" & No 2") and one with a 461 and a 12MHz crystal ("No3"). No1 & No2 has 47u and 0.1u over the supply lines.

No 1 and No 2 behaves the same. When connected to a powered usb-hub all is fine. When connected directly to the PC it doesn't work:



The 461-based design works i both cases. No 2 doesn't work when connected to a laptop.
No 1 & 2 uses two 1N4148 in the USB power line to get correct power. When connected directly to PC it measures 3.52V at the MCU power pins. Pin 5 (D-) = 3.18V, Pin 7 (D+) = 0.05V.
When connected to the hub: 3.49V at power pins, Pin 5 = 3.17 and Pin 7 = 0.00V.

So the power seems to be at the same levels. One thought is that the USB power from the computer is noisy, but is it noisy both in the PC and the laptop? Wish I had an oscillscope nearby...

Anyone has any ideas on what the problem is and how to fix it?

I had a similar problem.

viewtopic.php?f=8&t=4149

Turns out my zeners ( surface mounts ) where the problem. I put my through hole zeners in place and it now works direct to the PC. My readings were 4.0 volt though.

Hmm ok. My zener are hole-mounted, and reads 3.17. Yours read 4.0. They are rated at 3.6... But this the nature of zenerdiods, they are unpredictable. But at the same time, mine reads practicle the same in both cases. Strange indeed.