Renesas SH7781 - Manual (Page 576)

12. DDR2-SDRAM Interface (DBSC2)
Rev.1.00 Jan. 10, 2008 Page 546 of 1658
REJ09B0261-0100
write
MCK
Command
Data
MCKE
MODT
Terminating
resistor
in SDRAM
read
The above figure shows an example when a product with CL = 5 is used. As the ODT control signal (MODT)
is extended for one cycle, the product with CL = 5 or greater is required. If a product with CL = 4 or less is
used, MODT needs to be asserted before the issue of the WRITE command, which is not supported by this
LSI.
The interval between the READ and WRITE commands required for the SDRAM is assumed to be 4 cycles.
However, if the interval is 4 cycles, read data exists on the data bus when the terminating resistor in the
SDRAM is turned on since MODT is extended for one cycle. To prevent this, the interval should be set to 5
cycles.
5 cycles for
product with CL = 5
4 -> 5 cycles
tAOND
= 2 cycles
Extended for 1 cycle
tAOFD
= 2.5 cycles
If the interval from the READ command to the WRITE command is 4 cycles, read data exists
on the data bus when Rtt is turned on. Therefore, the interval should be 5 cycles.
Resistor ON
4 cycles for
product with CL = 5
High level
Figure 12.22 Important Information on One-Cycle Extension of ODT Control Signal
12.5.10 DDR2-SDRAM Power Supply Backup Function
The SDRAM power supply backup function utilizes the SDRAM self-refresh state to turn off the
power supply to most of the modules including the DBSC2 (all modules except the 1.8 V I/O),
while maintaining the data in the SDRAM. By using this function, not only is it possible to cut
power consumption, but the time needed to transfer data once again to the SDRAM can be
eliminated, since the valid data is maintained within the SDRAM (see figure 12.23). In order to
realize this function, in addition to the LSI device containing the DBSC2, a separate external
control circuit (microcomputer or similar) is needed to monitor the states of this LSI and the
memory.