Datasheet

13 SelfRefresh Entry command
(RAS# = "L", CAS# = "L", WE# = "H", CKE = "L", A0-A11 = Don't care)
The SelfRefresh is another refresh mode available in the SDRAM. It is the preferred refresh mode for
data retention and low power operation. Once the SelfRefresh command is registered, all the inputs to the
SDRAM become "don't care" with the exception of CKE, which must remain LOW. The refresh
addressing and timing is internally generated to reduce power consumption. The SDRAM may remain in
SelfRefresh mode for an indefinite period. The SelfRefresh mode is exited by restarting the external clock
and then asserting HIGH on CKE (SelfRefresh Exit command).
14 SelfRefresh Exit command
This command is used to exit from the SelfRefresh mode. Once this command is registered, NOP or
Device Deselect commands must be issued for t
XSR
(min.) because time is required for the completion of
any bank currently being internally refreshed. If auto refresh cycles in bursts are performed during normal
operation, a burst of 4096 auto refresh cycles should be completed just prior to entering and just after
exiting the SelfRefresh mode.
15 Clock Suspend Mode Entry / PowerDown Mode Entry command (CKE = "L")
When the SDRAM is operating the burst cycle, the internal CLK is suspended (masked) from the
subsequent cycle by issuing this command (asserting CKE "LOW"). The device operation is held intact
while CLK is suspended. On the other hand, when all banks are in the idle state, this command performs
entry into the PowerDown mode. All input and output buffers (except the CKE buffer) are turned off in the
PowerDown mode. The device may not remain in the Clock Suspend or PowerDown state longer than the
refresh period (64ms) since the command does not perform any refresh operations.
16 Clock Suspend Mode Exit / PowerDown Mode Exit command (CKE= "H")
When the internal CLK has been suspended, the operation of the internal CLK is reinitiated from the
subsequent cycle by providing this command (asserting CKE "HIGH", the command should be NOP or
deselect). When the device is in the PowerDown mode, the device exits this mode and all disabled buffers
are turned on to the active state. t
PDE
(min.) is required when the device exits from the PowerDown mode.
Any subsequent commands can be issued after one clock cycle from the end of this command.
17 Data Write / Output Enable, Data Mask / Output Disable command (DQM = "L", "H")
During a write cycle, the DQM signal functions as a Data Mask and can control every word of the input
data. During a read cycle, the DQM functions as the controller of output buffers. DQM is also used for
device selection, byte selection and bus control in a memory system.
AS4C4M16SA-C&I
Confidential
- 17 of 56 -
Rev.5.0 October 2018