Hi Pravin! If you are interested in interfacing USB controllers to 8051, I would suggest to forget it. As I am doing now a project that involves USB (mp3 player), I 've searched very much for USB controllers. For me the best solution is to look for 8051 uC that have USB controllers, than searching for different USB chips. Apart from Atmel, take a look at SMSC, Texas Instruments, ST and Cypress. Especially SMSC, TI and Cypress have high speed USB chips (USB 2.0) with 8051 core. For my mp3 player project I have chosen Cypress EZ-USB FX2 uC. You 'll find out that Cypress has very good literature and examples to start with USB. I don't mean that seperate USB controllers aren't good, but they are very hard to program. Unfortunately USB or FireWire aren't as easy as RS232 communication or parallel communication in the form of 8255 or Z80PIO peripherals. What's your project anyway, that has to do with USB?

Best regards,
George Pallis