I have written a firmware to download bulk data from my pc to an onboard flash which is interfaced using a cypress EzUSB-Fx chip(CY7C64613). I am using bulk endpoint 2 to send the 64 byte packets from the pc to the board. As soon as I setup up 64 bytes in the OUT2BUF on the host (PC) and set OUT2BC to 64, an interrupt is supposed to be raised, and my firmware has to enter the Interrupt Service Routine(ISR), from the while(1) loop where it indefinitely waits. The Keil C51 allows a function to be declared as an ISR using the 'interrupt' keyword along with the interrupt no. (in the range from 0 to 31) for which it is meant. I am unable to find what this interrupt no. would be for Bulk endpoint 2 (i.e. EP2OUT). The vector address for EP2OUT as per the datasheet is Ox2C while the "ezusb.h" header file contains the following line: #define OUT2BUF_USBVECT (11 << 2) usage of which gives an error at compile time. The Keil Application Note 103 which is supposed to talk abt 8051 interrupt vectors does provide these nos. but none of them corresponds to vector address 0x002C. Does anyone have any idea about this? Thanks in anticipation.