Memory Controller User's Guide

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4.6 Peripheral Bus Burst Priority Register (PBBPR)
DDR2 Memory Controller Registers
The peripheral bus burst priority register (PBBPR) helps prevent command starvation within the DDR2
memory controller. To avoid command starvation, the DDR2 memory controller momentarily raises the
priority of the oldest command in the command FIFO after a set number of transfers have been made.
The PR_OLD_COUNT bit sets the number of transfers that must be made before the DDR2 memory
controller raises the priority of the oldest command. The PBBPR is shown in Figure 24 and described in
Table 30 . See Section 2.8.2 for more details on command starvation.
Figure 24. Peripheral Bus Burst Priority Register (PBBPR)
31 16
Reserved
R-0
15 8 7 0
Reserved PR_OLD_COUNT
R-0 R/W-FFh
LEGEND: R/W = Read/Write; R = Read only; - n = value after reset
Table 30. Peripheral Bus Burst Priority Register (PBBPR) Field Descriptions
Bit Field Value Description
31-8 Reserved 0 Reserved
7-0 PR_OLD_COUNT 0-FFh Priority raise old counter. Specifies the number of memory transfers after which the DDR2
memory controller will elevate the priority of the oldest command in the command FIFO. Setting
this field to FFh disables this feature, thereby allowing old commands to stay in the FIFO
indefinitely.
0 1 memory transfer
1 2 memory transfers
2 3 memory transfers
3-FEh 4 to 255 memory transfers
FFh Feature disabled, commands may stay in command FIFO indefinitely
48 DDR2 Memory Controller SPRUEM4A – November 2007
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