Reference manual of i.MX6UL specifies following Boot modes.
First of all (on fresh board), we have downloaded the image through USB by putting the board in "Serial Downloader" mode.
After this we changed mode "Internal Mode" by which its booting the code from eMMC and executing code by dumping in DDR3.
However when we try to bring it again on " Serial Downloader" by making "01", its not going in to it.
This is happening for all the boards.