SH7785 Datasheet, PDF (577/1692 Pages) Renesas Technology Corp – 32-Bit RISC Microcomputer

12. DDR2-SDRAM Interface (DBSC2)
Normal operation
This LSI
DBSC2
IO cell
Internal
CKE
MCKE
SDRAM
Normal
operation
1.0-V
power on
1.8-V
power on
MBKPRST
External
High level _device
input
Status signal
When SDRAM power supply backup function is used
This LSI
DBSC2
IO cell
Data is retained
in self-refresh state
SDRAM
Internal
CKE
1.0-V
power off
1.8-V
power on
MCKE
Low level
output
MBKPRST
Low level
input
Data
retained
External
device
Status signal
As the power to most modules, except the 1.8-V
power supply for DDRPAD, is turned off, chip
power consumption can be reduced.
Figure 12.23 SDRAM Power Supply Backup Function
In order to implement the power supply backup function, a control signal MBKPRST is necessary
to hold MCKE at low level even when power other than for the 1.8 V I/O is turned off. When this
signal is at low level, MCKE pin can be held at low level even when the power supply within the
chip is in the turned-off state. After using the DBSC2 to put the SDRAM into the self-refresh
state, by using this MBKPRST signal to hold the MCKE signal at low level, the SDRAM self-
refresh state can be maintained even when the power supply in the chip is turned off.
To cancel the power supply backup state, perform a power-on reset. As a result, the DBSC2
registers are initialized, and so the self-refresh control circuit is also initialized. In order to put the
SDRAM into the self-refresh state before power-on reset, when the internal CKE signal is
indefinite, and also during power-on reset, the MBKPRST signal must be held at low level.
Power-on reset causes the DBSC2 to fix the internal CKE signal at low level, so that after power-
on reset is released the MBKPRST signal is raised to high level. (If not in the power supply
backup state, MBKPRST is always at high level and there is no problem.)
Thus the power supply backup state is cancelled through power-on reset, and so the software must
decide whether the normal SDRAM initialization sequence is necessary, or whether the LSI was in
the power supply backup state. In order to perform this decision, a state signal is input to this LSI
from an external control circuit. The method of input is arbitrary, and a general port can be used.
After power-on reset, the software monitors the state signal applied by the external control circuit,
and judges whether the state should be the power supply backup state or whether SDRAM
initialization is necessary. Before using register settings to send MCKE to high level, the state
signal must be made to signify a state other than the power supply backup state. (After driving pin
Rev.1.00 Jan. 10, 2008 Page 547 of 1658
REJ09B0261-0100