Intel
®
81341 and 81342 I/O Processors
December 2007
Developer’s Manual
Order Number: 315037-002US
591
DDR SDRAM Memory Controller—Intel
®
81341 and 81342
The DMCU logic determines the hit/miss status for reads and writes. For a new
DDR SDRAM transaction, the DMCU compares the address of the current transaction
with the address stored in the appropriate page address register. Assuming 512 Mbit
DDR2 SDRAM devices and two banks, there are eight pages kept open simultaneously.
The DDR SDRAM chip enables (
CS[1:0]#
) and leaf selects (
BA[2:0]
) determine which
page address to compare. BA2 is not available on 512 Mbit DDR2 SDRAM devices. For
1 Gbit/2 Gbit DDR2 SDRAM devices and two banks, there are sixteen pages kept open
simultaneously. The DDR SDRAM chip enables (
CS[1:0]#
) and leaf selects (
BA[2:0]
)
determine which page address to compare.
When the current transaction misses the open page selected then the DMCU closes the
open page pointed to by
CS[1:0]#
and
BA[2:0]
by issuing a
precharge
command. The
DMCU opens the current page with a
row-activate
command and the transaction
completes with a
read
or
write
command. When the DMCU opens the current page,
AD[35:13]
from the corresponding memory transaction queue is stored in the page
address register pointed to by
CS[1:0]#
and
BA[2:0]
so it may be compared for
future transactions.
For a page hit, the DMCU does not need to open the page (assert
RAS#
) and avoids
the RAS-to-CAS delay achieving greater performance. For a page hit, the two cycles
required for row activation are saved resulting in lower first word write latency.
When the current transaction hits the open page, then the page is already active and
the
read
or
write
command may be issued without a
row-activate
command. When the
next transaction is the same command type as the current transaction, and also a page
hit, the DMCU does not need to issue the command again, but simply drive column
address for an open page.
When the refresh timer expires and the DMCU issues an
auto-refresh
command, all
pages are closed and the valid bits are cleared.