DDR SDRAM Clocking and Interface Timing
MSC8144E Reference Manual, Rev. 3
Freescale Semiconductor
12-19
12.3.4.2 DDR SDRAM Refresh and Power-Saving Modes
In full-on mode, the DDR memory controller supplies the normal auto refresh to SDRAM. In
sleep mode, the DDR memory controller can be configured to take advantage of self-refreshing
SDRAMs or to provide no refresh support. Self-refresh support is enabled by the
DDR_SDRAM_CFG[SREN] bit.
Note:
In absence of refresh support, system software must preserve DDRS DRAM data (for
example - by copying the memory content to a non-volatile memory - disk, flash etc.)
before entering power saving mode.
The dynamic power-saving mode uses the
CKE
pin to power down the memory device
dynamically when there is no system memory activity. The
CKE
pin is deasserted when both
conditions are met
no memory refreshes are scheduled.
no memory accesses are scheduled.
The
CKE
pin is reasserted when a new access or refresh is scheduled or the dynamic power mode
is disabled. This mode is controlled with DDR_SDRAM_CFG[DYN_PWR].
Dynamic power management mode offers tight control of the memory system power
consumption by trading power for performance through the use of
CKE
. Powering up the DDR
SDRAM when a new memory reference is scheduled causes an access latency penalty,
depending upon whether active or precharge power-down is used, along with the settings of
TIMING_CFG_0[ACT_PD_EXIT] and TIMING_CFG_0[PRE_PD_EXIT]. A penalty of one
cycle is shown in Figure 12-12.
Figure 12-12. DDR SDRAM Power-Down Mode
The entry and exit timing for self-refreshing SDRAMs in Sleep mode is shown in Figure 12-13
and Figure 12-14.
Memory Bus Clock
NOP
NOP
COMMAND
ACT
CKE
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