Let me add to Per Westermark's comments.

There is no doubt that when you snoop through the web that you will find some examples where folks have played around with bit-banging low speed USB 1.1 protocols. This has often been done on various flavors of AVR MCUs due in part to the work of someone who published some work like this on AVR Freaks some 5-7 years ago. BUT if you look at some of this stuff you will soon see that almost all of it is an implementation of a SUBSET of the USB protocol. I've personally seen web site after web site where people declare "It works!" but then suggest you come back later for the more mature version. Most times there is little additional activity in this area and they may even go so far as to suggest that "This may not be compatible with all USB hosts" but also like to sugarcoat their stuff with "No problems have been noted so far".

There is a reason why MCUs are available with onboard USB controllers. It is really the only way to get reliable send and receive of USB packets to and from the bus.

There is another thing to consider as well. Many folks look at a simple USB device, see only see two connection wires, and think that therefore it must be oh so simple. BUT note that even when you use an MCU that has an onboard USB controller there is much work that the MCU has to do in firmware in order to get the USB protocols to work. Walk through a USB MCU implementation without a ready made example and a good appnote and you'll soon see how much work it actually is.

Michael Karas