W9425G6DH
Publication Release Date:Feb. 12 , 2008
- 11 - Revision A8
7.2.11 Burst Read Stop Command
(RAS = "H", CAS= "H", WE = "L")
The Burst stop command is used to stop the burst operation. This command is only valid during a
Burst Read operation.
7.2.12 Device Deselect Command
(CS = "H")
The Device Deselect command disables the command decoder so that the RAS , CAS ,
WE and Address inputs are ignored. This command is similar to the No-Operation command.
7.2.13 Auto Refresh Command
(RAS = "L", CAS = "L", WE = "H", CKE = "H", BS0, BS1, A0 to A12 = Don’t Care)
AUTO REFRESH is used during normal operation of the DDR SDRAM and is analogous to CAS–
BEFORE–RAS (CBR) refresh in previous DRAM types. This comm and is non persistent, so it
must be issued each time a refre sh i s re quired. The refresh addressing is generated by the
internal refresh controller. This ma kes the address bits ”Don’t Care” during an AUTO REFRES H
command. The DDR SDRAM requires AUTO REFRESH cycles at an average periodic interval of
tREFI (maximum). To allow for improved efficiency in scheduling and switching between tasks,
some flexibility in the absolute refresh interval is provided. A maximum of eight AUTO REFRESH
commands can be posted to any given DDR SDRAM, and the maximum absolute interval
between any AUTO REFRESH command and the next AUTO REFRESH command is 8 * tREFI.
7.2.14 Self Refresh Entry Command
(RAS = "L", CAS = "L", WE = "H", CKE = "L", BS0, BS1, A0 to A12 = Don’t Care)
The SELF REFRESH command can be used to retain data in the DDR SDRAM, even if the rest of
the system is powered down. When in the self refresh mode, the DDR SDRAM retains data
without external clocking. The SELF REFRESH command is initiated like an AUTO REFRESH
command except CKE is disabled (LOW). The DLL is automatically disabled upon entering SELF
REFRESH, and is automatically enabled upon exiting SELF REFRESH. Any time the DLL is
enabled a DLL Reset must follow and 200 clock cycles should occur before a READ command
can be issued. Input signals except CKE are “Don’t Care” during SELF REFRESH. Since CKE is
an SSTL_2 input, VREF must be maintained during SELF REF RESH.
7.2.15 Self Refresh Exit Command
(CKE = "H", CS = "H" or CKE = "H", RAS = "H", CAS = "H")
The procedure for exiting self refresh requires a sequence of com mands. First, CLK must be
stable prior to CKE going back HIGH. Once CKE is HIGH, the DDR SDRAM must have NOP
commands issue d for tXSNR because time is required for the completion of any internal refresh in
progress. A simple algorithm for meeting both refresh and DLL requirements is to apply NOPs for
200 clock cycles before a pplying any other command.
The use of SELF REFREH mode introduces the possibility that an internally timed event can be
missed when CKE is raised for exit from self refresh mode. Upon exit from SELF REFRESH an
extra auto refresh command is recommended.