256Mb: x4, x8, x16 SDRAM
SELF REFRESH Operation
SELF REFRESH Operation
The self refresh mode can be used to retain data in the device, even when the rest of the
system is powered down. When in self refresh mode, the device retains data without ex-
ternal clocking. The SELF REFRESH command is initiated like an AUTO REFRESH com-
mand, except CKE is disabled (LOW). After the SELF REFRESH command is registered,
all the inputs to the device become “Don’t Care” with the exception of CKE, which must
remain LOW.
After self refresh mode is engaged, the device provides its own internal clocking, ena-
bling it to perform its own AUTO REFRESH cycles. The device must remain in self re-
fresh mode for a minimum period equal to tRAS and remains in self refresh mode for an
indefinite period beyond that.
The procedure for exiting self refresh requires a sequence of commands. First, CLK
must be stable prior to CKE going back HIGH. (Stable clock is defined as a signal cycling
within timing constraints specified for the clock ball.) After CKE is HIGH, the device
must have NOP commands issued for a minimum of two clocks for tXSR because time is
required for the completion of any internal refresh in progress.
Upon exiting the self refresh mode, AUTO REFRESH commands must be issued accord-
ing to the distributed refresh rate (tREF/refresh row count) as both SELF REFRESH and
AUTO REFRESH utilize the row refresh counter.
PDF: 09005aef8091e6d1
256Mb_sdr.pdf - Rev. R 10/12 EN
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