Registers
412
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
DDR2/mDDR Memory Controller
14.4.7 Peripheral Bus Burst Priority Register (PBBPR)
The peripheral bus burst priority register (PBBPR) helps prevent command starvation within the
DDR2/mDDR memory controller. To avoid command starvation, the DDR2/mDDR 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/mDDR memory controller raises the priority of the oldest command. See
for more details on command starvation.
Proper configuration of the PBBPR is critical to correct system operation. The DDR2/mDDR memory
controller always prioritizes accesses to open rows as highest, if there is any bank conflict regardless of
master priority. This is done to allow most efficient utilization of the DDR2/mDDR. However, it could lead
to excessive blocking of high priority masters. If the PR_OLD_COUNT bits are cleared to 00h, then the
DDR2/mDDR memory controller always honors the master priority, regardless of open row/bank status.
For most systems, the PBBPR should be set to a moderately low value to provide an acceptable balance
of DDR2/mDDR efficiency and latency for high priority masters (for example, 10h or 20h).
The PBBPR is shown in
and described in
.
Figure 14-26. 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 14-29. Peripheral Bus Burst Priority Register (PBBPR) Field Descriptions
Bit
Field
Value
Description
31-8
Reserved
0
Any writes to these bit(s) must always have a value of 0.
7-0
PR_OLD_COUNT
0-FFh
Priority raise old counter. Specifies the number of memory transfers after which the
DDR2/mDDR memory controller will elevate the priority of the oldest command in the command
FIFO. Clearing to 00h will ensure master priority is strictly honored (at the cost of decreased
DDR2/mDDR memory controller efficiency, as open row will always be closed immediately if
any bank conflict occurs). Recommended setting for typical system operation is between 10h
and 20h.
0
1 memory transfer
1h
2 memory transfers
2h
3 memory transfers
3h-FFh
4 to 256 memory transfers