Quote Originally Posted by kirtcathey View Post
... but why is speed an issue? People can only type so fast.
Well, let's say that an 18F is running at 48Mhz. (the fastest 8-bit PIC)
Divide that by 4, and the CPU core will execute 12 million instructions per second.

Full-speed USB is 12 million bits per second. So at best, you could only execute 1 instruction of your program per incoming BIT of data.

Now, considering it takes 4 instructions to copy the state of a pin to a BIT variable. It's easy to see that it just can't be done with software.

Plus there is a LOT of overhead communication with USB. Even if nobody was typing on the keyboard, there's still a ton of data being passed back and forth. The computer is constantly asking the device ... "Are you there", and the device says ... "Yup, I'm here", over and over and ...
When it gets unplugged and the computer doesn't get an "I'm here" anymore, it disconnects the device driver and you hear the ding-dong.

So basically, USB interfaces MUST be done with hardware.

hth,