I encounter problem with a USB 1.1 device design. When it is plugged to a particular PC, the device fails to be recognised. Or even at times when it is recognised, it can vanish all in a sudden. The same device works perfectly on all other PCs within scope.
The problem is somehow solved by adding a 100pF capacitor at the DM or DP pin (either will do) to ground. Since i have never seen a schematic of USB that has a de-coupling capacitor at this position, i suspect it is a inherent noise problem caused by the PCB?
Would it be a problem related to layout geometry? Or would it be merely noise-related?
This PC (the host controller) could be funnier than the others as my device works fine on most other PCs. But other off-the-shelf devices like flash disks also work well on this PC. So i think there is some compatibility issue to improve here.