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RAS Down Mode and RAS Up Mode: Even when burst operation is selected, it may happen that
access to DRAM space is not continuous, but is interrupted by access to another space. In this
case, if the RAS signal is held low during the access to the other space, burst operation can be
resumed when the same row address in DRAM space is accessed again.
• RAS down mode
To select RAS down mode, set the RCDM bit in MCR to 1. If access to DRAM space is
interrupted and another space is accessed, the RAS signal is held low during the access to the
other space, and burst access is performed if the row address of the next DRAM space access
is the same as the row address of the previous DRAM space access. Figure 4-21 shows an
example of the timing in RAS down mode.
Note, however, that the RAS signal will go high if a refresh operation interrupts RAS down
mode.
ø
A
to A
23
0
CSn (RAS)
CAS, LCAS
D
to D
15
0
Note: n = 2 to 5
Figure 4-21 Example of Operation Timing in RAS Down Mode
92
DRAM access
T
T
T
p
r
c1
External space
access
T
T
T
c2
1
2
DRAM access
T
T
c1
c2
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