AS4C64M8D2
Refresh command
When CS#, RAS# and CAS# are held LOW and WE# HIGH at the rising edge of the clock, the chip enters the
Refresh mode (REF). All banks of the DDR2 SDRAM must be precharged and idle for a minimum of the
Precharge time (tRP) before the Refresh command (REF) can be applied. An address counter, internal to the
device, supplies the bank address during the refresh cycle. No control of the external address bus is required
once this cycle has started.
When the refresh cycle has completed, all banks of the DDR2 SDRAM will be in the precharged (idle) state. A
delay between the Refresh command (REF) and the next Activate command or subsequent Refresh command
must be greater than or equal to the Refresh cycle time (tRFC).To allow for improved efficiency in scheduling and
switching between tasks, some flexibility in the absolute refresh interval is provided. A maximum of eight Refresh
commands can be posted to any given DDR2 SDRAM, meaning that the maximum absolute interval between any
Refresh command and the next Refresh command is 9 * tREFI
.
Self refresh operation
The Self Refresh command can be used to retain data in the DDR2 SDRAM, even if the rest of the system is
powered down. When in the Self Refresh mode, the DDR2 SDRAM retains data without external clocking. The
DDR2 SDRAM device has a built-in timer to accommodate Self Refresh operation. The Self Refresh Command is
defined by having CS#, RAS#, CAS# and CKE# held LOW with WE# HIGH at the rising edge of the clock. ODT
must be turned off before issuing Self Refresh command, by either driving ODT pin LOW or using EMRS
command. Once the Command is registered, CKE must be held LOW to keep the device in Self Refresh mode.
The DLL is automatically disabled upon entering Self Refresh and is automatically enabled upon exiting Self
Refresh. When the DDR2 SDRAM has entered Self Refresh mode all of the external signals except CKE, are
“don’t care”. For proper Self Refresh operation all power supply pins (VDD, VDDQ, VDDL and VREF) must be at valid
levels. The DRAM initiates a minimum of one refresh command internally within tCKE period once it enters Self
Refresh mode. The clock is internally disabled during Self Refresh Operation to save power. The minimum time
that the DDR2 SDRAM must remain in Self Refresh mode is tCKE. The user may change the external clock
frequency or halt the external clock one clock after Self Refresh entry is registered, however, the clock must be
restarted and stable before the device can exit Self Refresh operation.
The procedure for exiting Self Refresh requires a sequence of commands. First, the clock must be stable prior to
CKE going back HIGH. Once Self Refresh Exit is registered, a delay of at least tXSNR must be satisfied before a
valid command can be issued to the device to allow for any internal refresh in progress. CKE must remain HIGH
for the entire Self Refresh exit period tXSRD for proper operation except for Self Refresh re-entry. Upon exit from
Self Refresh, the DDR2 SDRAM can be put back into Self Refresh mode after waiting at least tXSNR period and
issuing one refresh command(refresh period of tRFC). NOP or Deselect commands must be registered on each
positive clock edge during the Self Refresh exit interval tXSNR. ODT should be turned off during tXSRD. The use of
Self Refresh mode introduces the possibility that an internally timed refresh event can be missed when CKE is
raised for exit from Self Refresh mode. Upon exit from Self Refresh, the DDR2 SDRAM requires a minimum of
one extra auto refresh command before it is put back into Self Refresh mode.
Confidential
18
Rev. 1.0
Feb. /2014
ALSC [ ALLIANCE SEMICONDUCTOR CORPORATION ]
