Editing Synchronous dynamic random-access memory (section)

== Low power modes ==
As mentioned, the clock enable (CKE) input can be used to effectively stop the clock to an SDRAM. The CKE input is sampled each rising edge of the clock, and if it is low, the following rising edge of the clock is ignored for all purposes other than checking CKE. As long as CKE is low, it is permissible to change the clock rate, or even stop the clock entirely.

If CKE is lowered while the SDRAM is performing operations, it simply "freezes" in place until CKE is raised again.

If the SDRAM is idle (all banks precharged, no commands in progress) when CKE is lowered, the SDRAM automatically enters power-down mode, consuming minimal power until CKE is raised again. This must not last longer than the maximum refresh interval t<sub>REF</sub>, or memory contents may be lost. It is legal to stop the clock entirely during this time for additional power savings.

Finally, if CKE is lowered at the same time as an auto-refresh command is sent to the SDRAM, the SDRAM enters self-refresh mode. This is like power down, but the SDRAM uses an on-chip timer to generate internal refresh cycles as necessary. The clock may be stopped during this time. While self-refresh mode consumes slightly more power than power-down mode, it allows the memory controller to be disabled entirely, which commonly more than makes up the difference.

SDRAM designed for battery-powered devices offers some additional power-saving options. One is temperature-dependent refresh; an on-chip temperature sensor reduces the refresh rate at lower temperatures, rather than always running it at the worst-case rate. Another is selective refresh, which limits self-refresh to a portion of the DRAM array. The fraction which is refreshed is configured using an extended mode register. The third, implemented in [[Mobile DDR]] (LPDDR) and LPDDR2 is "deep power down" mode, which invalidates the memory and requires a full reinitialization to exit from. This is activated by sending a "burst terminate" command while lowering CKE.