Embedded Design Handbook

ID 683689
Date 8/28/2023
Public
Document Table of Contents

3.5.5.7.4. Bursting

SDRAM devices employ bursting to improve throughput. Bursts group a number of transactions to sequential addresses, allowing data to be transferred back-to-back without incurring the overhead of requests for individual transactions. If you are using the high performance DDR/DDR2 SDRAM controller, you may be able to take advantage of bursting in the system interconnect fabric as well. Bursting is only useful if both the master and slave involved in the transaction are burst-enabled. Refer to the documentation for the master in question to check whether bursting is supported.

Selecting the burst size for the high performance DDR/DDR2 SDRAM controller depends on the mode in which you use the controller. In half-rate mode, the Avalon-MM data port is four times the width of the actual SDRAM device; consequently, four transactions are initiated to the SDRAM device for each single transfer in the system interconnect fabric. A burst size of four is used for those four transactions to SDRAM. This is the maximum size burst supported by the high performance DDR/DDR2 SDRAM controller. Consequently, using bursts for the high performance DDR/DDR2 SDRAM controller in half-rate mode does not increase performance because the system interconnect fabric is already using its maximum supported burst-size to carry out each single transaction.

However, in full-rate mode, you can use a burst size of two with the high performance DDR/DDR2 SDRAM controller. In full-rate mode, each Avalon transaction results in two SDRAM device transactions, so two Avalon transactions can be combined in a burst before the maximum supported SDRAM controller burst size of four is reached.