===== USB mode: USB1 or USB2; low speed, full speed or high speed =====
For sampling, I speed up five times the saving speed on RAM. I succeed sampling an USB1 transmission: "Logitech dual action USB joystick", and an USB2: "Sony PS3 USB joystick". PS3 joystick is not stable enough with my FPGA, but Logitech joystick is correct.
http://github.com/renaudhelias/CoreAmstrad/blob/master/BuildYourOwnZ80Computer/USB_logitech.vhd
http://www.youtube.com/watch?v=2zEp1tHroBs
http://github.com/renaudhelias/CoreAmstrad/blob/master/BuildYourOwnZ80Computer/USB_ps3.vhd
http://www.youtube.com/watch?v=fh4v4OXridc
USB is just a state machine (welcome how are you today, show me your state, show me your state, show me your state....), encoding (have to read USB manual), your can use some usb snifer software to decode them. Snifer software does not show low level message (ack ko ok) but does show the high level message (ones that show that a button is pressed or not)
As it is just encoding, you can capture signals and show that they differ only when you do unpress or press a button.
===== pull up and pull down =====