ddr burst ,avalon mm interface

Are you creating your own master to use in SOPC Builder/Qsys? If so you don't need to worry about driving this signal. The fabric will generate the burst begin at the appropriate time for you.

If you are using the memory controller in a stand alone fashion with an Avalon-MM local interface, you should strobe the burst begin even if you drive a burst length of 1 into the memory controller. Also the burstbegin signal does not heed waitrequest (local_ready == 0) so if the memory controller is not ready you shouldn't send another burstbegin. To learn more refer to the Avalon specification: http://www.altera.com/literature/manual/mnl_avalon_spec.pdf