3069RF-ZTAT Datasheet, PDF (189/1003 Pages) Renesas Technology Corp – RENESAS 16-BIT SINGLE-CHIP MICROCOMPUTER

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.
DRAM access
DRAM access
External space
access
Tp
Tr
Tc1
Tc2
Tc1
Tc2
T1
T2
φ
A23 to A0
AS
CSn(RAS)
(UC^PA^BS⁴//L^PC^BA⁵ S)
D15 to D0
Note: n = 2 to 5
Figure 6.25 Example of Operation Timing in RAS Up Mode
Rev. 5.0, 09/04, page 167 of 978