Texas Instruments TMS320DM647, DM648 DSP manual Command Starvation

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Peripheral Architecture

Next, the DDR2 memory controller examines each of the commands selected by the individual masters and performs the following reordering:

∙Among all pending reads, selects reads to rows already open. Among all pending writes, selects writes to rows already open.

∙Selects the highest priority command from pending reads and writes to open rows. If multiple commands have the highest priority, then the DDR2 memory controller selects the oldest command.

The DDR2 memory controller may now have a final read and write command. If the Read FIFO is not full, then the read command will be performed before the write command, otherwise the write command will be performed first.

Besides commands received from on-chip resources, the DDR2 memory controller also issues refresh commands. The DDR2 memory controller attempts to delay refresh commands as long as possible to maximize performance while meeting the SDRAM refresh requirements. As the DDR2 memory controller issues read, write, and refresh commands to DDR2 SDRAM device, it follows the following priority scheme:

1.(Highest) Refresh request resulting from the Refresh Must level of urgency (see Section 2.8) being reached

2.Read request without a higher priority write (selected from above reordering algorithm)

3.Refresh request resulting from the Refresh Need level of urgency (see Section 2.8) being reached

4.Write request (selected from above reordering algorithm)

5.Refresh request resulting from Refresh May level of urgency (see Section 2.8) being reached

6.(Lowest) Request to enter self-refresh mode

The following results from the above scheduling algorithm:

∙All writes from a single master will complete in order

∙All reads from a single master will complete in order

∙From the same master, any read to the same location (or within 2048 bytes) as a previous write will complete in order

2.7.2Command Starvation

The reordering and scheduling rules listed above may lead to command starvation, which is the prevention of certain commands from being processed by the DDR2 memory controller. Command starvation results from the following conditions:

∙A continuous stream of high-priority read commands can block a low-priority write command

∙A continuous stream of DDR2 SDRAM commands to a row in an open bank can block commands to the closed row in the same bank.

To avoid these conditions, the DDR2 memory controller can momentarily raise the priority of the oldest command in the command FIFO after a set number of transfers have been made. The PRIO_RAISE field in the Burst Priority Register (BPRIO) sets the number of the transfers that must be made before the DDR2 memory controller will raise the priority of the oldest command.

Note: Leaving the PRIO_RAISE bits at their default value (FFh) disables this feature of the DDR2 memory controller. This means commands can stay in the command FIFO indefinitely. Therefore, these bits should be set to FEh immediately following reset to enable this feature with the highest level of allowable memory transfers. It is suggested that system-level prioritization be set to avoid placing high-bandwidth masters on the highest priority levels. These bits can be left as FEh unless advanced bandwidth/prioritization control is required.

24 DSP DDR2 Memory Controller SPRUEK5A –October 2007

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