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Section 6 Bus Controller
When RAS down mode is selected, the CAS-before-RAS refresh function provided with this
DRAM interface must always be used as the DRAM refreshing method. When a refresh
operation is performed, the RAS signal goes high immediately beforehand. The refresh interval
setting must be made so that the maximum DRAM RAS pulse width specification is observed.
When the self-refresh function is used, the RDM bit must be cleared to 0, and RAS up mode
selected, before executing a SLEEP instruction in order to enter software standby mode. Select
RAS down mode again after exiting software standby mode.
Note that RAS down mode cannot be used when HWR and LWR are selected for UCAS and
LCAS, a device other than DRAM is connected to external space, and HWR and LWR are
used as write strobes.
• RAS Up Mode
To select RAS up mode, clear the RDM bit to 0 in DRCRA. Each time access to DRAM space
is interrupted and another space is accessed, the RAS signal returns to the high level. Burst
operation is only performed if DRAM space is continuous. Figure 6.25 shows an example of
the timing in RAS up mode.
External space
DRAM access
DRAM access
access
T
Tr
T
T
T
T
T
T
p
c1
c2
c1
c2
1
2
φ
A
to A
23
0
AS
CSn(RAS)
PB
/PB
4
5
(UCAS/LCAS)
D
to D
15
0
Note: n = 2 to 5
Figure 6.25 Example of Operation Timing in RAS Up Mode
Rev. 4.00 Jan 26, 2006 page 179 of 938
REJ09B0276-0400
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