AS4C256M16D3
Input Clock frequency change
Once the DDR3 SDRAM is initialized, the DDR3 SDRAM requires the clock to be “stable” during almost all
states of normal operation. This means once the clock frequency has been set and is to be in the “stable state”,
the clock period is not allowed to deviate except for what is allowed for by the clock jitter and SSC (spread
spectrum clocking) specification.
The input clock frequency can be changed from one stable clock rate to another stable clock rate under two
conditions: (1) Self-Refresh mode and (2) Precharge Power-Down mode. Outside of these two modes, it is
illegal to change the clock frequency.
For the first condition, once the DDR3 SDRAM has been successfully placed in to Self-Refresh mode and
tCKSRE has been satisfied, the state of the clock becomes a don’t care. Once a don’t care, changing the clock
frequency is permissible, provided the new clock frequency is stable prior to tCKSRX. When entering and
exiting Self-Refresh mode of the sole purpose of changing the clock frequency, the Self-Refresh entry and exit
specifications must still be met. The DDR3 SDRAM input clock frequency is allowed to change only within the
minimum and maximum operating frequency specified for the particular speed grade.
The second condition is when the DDR3 SDRAM is in Precharge Power-Down mode (either fast exit mode or
slow exit mode). If the RTT_Nom feature was enabled in the mode register prior to entering Precharge power
down mode, the ODT signal must continuously be registered LOW ensuring RTT is in an off state. If the
RTT_Nom feature was disabled in the mode register prior to entering Precharge power down mode, RTT will
remain in the off state. The ODT signal can be registered either LOW or HIGH in this case. A minimum of
tCKSRE must occur after CKE goes LOW before the clock frequency may change. The DDR3 SDRAM input
clock frequency is allowed to change only within the minimum and maximum operating frequency specified for
the particular speed grade. During the input clock frequency change, ODT and CKE must be held at stable
LOW levels. Once the input clock frequency is changed, stable new clocks must be provided to the DRAM
tCKSRX before precharge Power Down may be exited; after Precharge Power Down is exited and tXP has
expired, the DLL must be RESET via MRS. Depending on the new clock frequency additional MRS commands
may need to be issued to appropriately set the WR, CL, and CWL with CKE continuously registered high.
During DLL re-lock period, ODT must remain LOW and CKE must remain HIGH. After the DLL lock time, the
DRAM is ready to operate with new clock frequency.
Confidential
35
Rev. 3.0
Aug. /2014
ALSC [ ALLIANCE SEMICONDUCTOR CORPORATION ]
