10 SDRAM CONTROLLER (SDRAMC)
S1C33L26 TECHNICAL MANUAL
Seiko Epson Corporation
10-13
Self-refresh
Self-refresh uses the self-refresh function of the SDRAM and does not require any clocks during the refresh pe-
riod, thus helping to reduce power consumption. This self-refresh function is also used for data retention in the
power-down mode. To cause the SDRAM to be self-refreshed, set SELEN/SDRAMC_REF register to 1. This
enables the SDRAMC to send the self-refresh command (which sets the SDCKE output to low) to the SDRAM.
The command is actually sent a certain time after accessing or auto-refreshing the SDRAM, so the SDRAMC
contains a self-refresh counter to count this time. The counter counts on SDCLK clock edges, and when the
designated count is reached, the SDRAMC sends the self-refresh command to the SDRAM. When an SDRAM
access or auto-refresh command is issued, the counter is reset and starts counting again. The counter value can
be specified in a range of 1 to 127 using the SELCO[6:0]/SDRAMC_REF register. Do not set the counter to 0
when the self-refresh function is enabled.
If an access to the SDRAM occurs while the SDRAM is in self-refresh mode, SDCKE is reset to high, thereby
self-refresh mode is canceled. After the SDRAM access has finished, the SDRAMC sends another self-refresh
command when the designated count is reached again. When the auto-refresh command is issued or an SDRAM
access occurs, the counter will restart if the self-refresh command has not been sent to the SDRAM. Therefore,
the self-refresh counter value to be set must be smaller than the auto-refresh counter value.
SELF
L
PALL
t
RP
SDCLK
Command
SDCKE
#SDCS
#SDRAS
#SDCAS
#SDWE
SDBA[1:0]
SDA10
SDA[12:11, 9:0]
DQMH/DQML
DQ[15:0]
SREFDO
Self-refresh mode
Enters self-refresh mode
Exits self-refresh mode
The SDRAM clock stops when SCKON = 0.
5.5.2 Self-Refresh
Figure 10.
During self-refresh (while SDCKE = low), the SREFDO/SDRAMC_REF register remains 0. Therefore, it is
possible to determine whether or not self-refresh is in operation by reading this status bit.
Furthermore, SDRAM clock output during self-refresh can be turned off in order to reduce power consumption
by setting the SCKON/SDRAMC_REF register to 0.
To cancel self-refresh mode
Perform the following procedure to cancel self-refresh mode:
1. Disable self-refresh mode by clearing SELEN to 0.
2. Read data from an SDRAM address (any address can be specified).
3. Read SREFDO to check if self-refresh mode is canceled (SREFDO = 1).
Note that auto-refresh mode cannot be canceled.
Note: Be sure to avoid setting SDON/SDRAMC_INIT register to 0 (SDRAMC disabled) during self-
refreshing. Before disabling the SDRAMC, always make sure the SDRAMC is not in self-refresh
mode.