Analyzing USB Comm

[kirtcathey]

Hi All.

This is something I have been working on a while, and as I discover more, the scope expands! I (still) want to create a device that connects between a keyboard and host PC that will trigger a remote process when the end-user types a certain string into the keyboard. I am currently using the 18F2550. Yes, I understand that the PIC 18F cannot be a USB host. I am not trying to host the USB keyboard but just want information that comes from it to the PC host.

This has led me to looking into using an onboard comparator tapped into the D+ and D- lines and decipher the key strokes with code. Which is a big plus for size, since I may not need a USB PIC afterall. I think I can setup the comparator with the following information, but do not know exactly how. For example, I think that a threshold voltage of 1.25 Volts is probably going to work, but do not know where to connect the D+ and D- lines (that are tapped). Also, how do I work out the timing and setup the timer? Will one comparator suffice, or do I need to setup two comparators?

Here is the information that I've got about the USB differential signaling:
Transmitted signal levels are 0.0–0.3 volts for low and 2.8–3.6 volts for high in Full Speed (FS) and Low Speed (LS) modes.

Now, if we go this far, I want to create something bigger than a keyboard detector! We can create another analysis tool for the workbench - a USB protocol analyzer.

Any help?

Thank you.