Intel
®
81341 and 81342 I/O Processors
December 2007
Developer’s Manual
Order Number: 315037-002US
617
DDR SDRAM Memory Controller—Intel
®
81341 and 81342
7.4
Power Failure Mode
The 81341 and 81342 is an I/O processor used in server applications including
networking and storage. Specifically, the storage applications supported utilize the
81341 and 81342 as the IOP for a SCSI RAID disk subsystem and the local memory is
used for disk caching. The local memory is used for the temporary storage of disk
writes which greatly improves disk performance.
While the host assumes all written data is stored on the non-volatile disk subsystem,
the IOP must ensure that eventually all the data in the disk cache is actually stored
onto disk.
The power supply could fail to provide power to the I/O subsystem in the case of a
power outage or a failed power supply. It is imperative that the cached data within the
IOPs local memory is not lost. When power fails, the local memory subsystem must
remain powered with a battery backup and some agent must continue to refresh at the
appropriate interval specified by the memory component datasheet.
This section defines a mechanism with which the 81341 and 81342 memory controller
ensures that the data within local memory is not lost during a power failure.
7.4.1
Theory of Operation
DDR SDRAM technology provides a simple way of enabling data preservation through
the
self-refresh
command. This command is issued by the memory controller and the
DDR SDRAM refreshes itself autonomously with internal logic and timers. The
self-refresh
command is defined in
.
The DDR SDRAM device remains in self-refresh mode as long as:
• The device continues to be powered.
•
CKE
is held low until the memory controller is ready to control the DDR SDRAM
once again.
Power to the DDR SDRAM subsystem is ensured with an adequate battery backup and a
reliable method for switching between system power and battery power. The memory
controller is responsible for deasserting
CKE[1:0]
when issuing the
self-refresh
command but while power gradually drops,
CKE[1:0]
must
remain deasserted
regardless of the
state of V
cc
powering the 81341 and 81342.