ESMT
M12L128168A
DEVICE OPERATIONS (Continued)
AUTO REFRESH
SELF REFRESH
The storage cells of SDRAM need to be refreshed every 64ms
to maintain data. An auto refresh cycle accomplishes refresh of
a single row of storage cells. The internal counter increments
automatically on every auto refresh cycle to refresh all the
rows. An auto refresh command is issued by asserting low on
CS , RAS and CAS with high on CKE and WE . The auto
refresh command can only be asserted with all banks being in
idle state and the device is not in power down mode (CKE is
high in the previous cycle). The time required to complete the
auto refresh operation is specified by tRFC(min). The minimum
number of clock cycles required can be calculated by driving
tRFC with clock cycle time and them rounding up to the next
higher integer. The auto refresh command must be followed by
NOP’s until the auto refresh operation is completed. The auto
refresh is the preferred refresh mode when the SDRAM is
being used for normal data transactions. The auto refresh
cycle can be performed once in 15.6us.
The self refresh is another refresh mode available in the
SDRAM. The self refresh is the preferred refresh mode for
data retention and low power operation of SDRAM. In self
refresh mode, the SDRAM disables the internal clock and
all the input buffers except CKE. The refresh addressing
and timing is internally generated to reduce power
consumption.
The self refresh mode is entered from all banks idle state
by asserting low on CS , RAS , CAS and CKE with
high on WE . Once the self refresh mode is entered, only
CKE state being low matters, all the other inputs including
clock are ignored to remain in the refresh.
The self refresh is exited by restarting the external clock
and then asserting high on CKE. This must be followed by
NOP’s for a minimum time of tRFC before the SDRAM
reaches idle state to begin normal operation. It is
recommended to use burst 4096 auto refresh cycles
immediately before and after self refresh.
Elite Semiconductor Memory Technology Inc.
Publication Date: Oct. 2006
Revision: 2.0
12/43
ESMT [ ELITE SEMICONDUCTOR MEMORY TECHNOLOGY INC. ]
