SH7616 Datasheet, PDF (364/935 Pages) Renesas Technology Corp – 32-Bit RISC Microcomputer SuperH™ RISC engine Family/SH7600 Series

Section 7 Bus State Controller (BSC)
specification for the DRAM used. First make the settings for RTCOR, RTCNT, and the RMODE
and RFSH bits in MCR, then make the CKS2–CKS0 and RRC2–RRC0 settings in RTCSR. When
the clock is selected by CKS2–CKS0, RTCNT starts counting up from the value at that time. The
RTCNT value is constantly compared with the RTCOR value, and when the two values match, a
refresh request is generated and the number of CAS-before-RAS refreshes set in bits RRC2–RRC0
are performed. At the same time, RTCNT is cleared to zero and the count-up is restarted. Figure
7.51 shows the CAS-before-RAS refresh cycle timing.
The number of RAS assert cycles in the refresh cycle is specified by bits TRAS1 and TRAS0 in
MCR. As with ordinary accesses, the specification of the RAS precharge time in the refresh cycle
follows the setting of bits TRP1 and TRP0 in MCR.
CKIO
Tp
Trr
Trc1
Trc2
Tre
RAS
CASn
RD/WR
RD
Figure 7.51 DRAM CAS-before-RAS Refresh Cycle Timing
Self-Refreshing: A self-refresh is started by setting both the RMODE bit and the RFSH bit to 1.
During the self-refresh, DRAM cannot be accessed. Self-refreshing is cleared by clearing the
RMODE bit to 0. Self-refresh timing is shown in figure 7.52. Settings must be made so that self-
refresh clearing and data retention are performed correctly, and CAS-before-RAS refreshing is
immediately performed at the correct intervals. When self-refreshing is started from the state in
which CAS-before-RAS refreshing is set, or when exiting standby mode by means of a manual
reset or NMI, auto-refreshing is restarted if RFSH is set to 1 and RMODE is cleared to 0 when
self-refresh mode is cleared. If the transition from clearing of self-refresh mode to starting auto-
refresh takes time, this time should be taken into consideration when setting the initial value of
RTCNT. When the RTCNT value is set to RTCOR-1, the refresh can be started immediately.
Rev. 2.00 Mar 09, 2006 page 338 of 906
REJ09B0292-0200