HD6417751 Datasheet, PDF (430/1105 Pages) Renesas Technology Corp – SuperH RISC engine

CKIO
TRr1 TRr2 TRr3 TRr4 TRr5 Trc Trc Trc
A25–A0
RD/
D31–D0
Figure 13.21 DRAM CAS-Before-RAS Refresh Cycle Timing (TRAS = 0, TRC = 1)
• Self-Refresh
The self-refreshing supported by the SH7751 Series is shown in figure 13.22.
After the self-refresh is cleared, the refresh controller immediately generates a refresh request.
The RAS precharge time immediately after the end of the self-refreshing can be set by bits
TRC2–TRC0 in MCR.
DRAMs include low-power products (L versions) with a long refresh cycle time (for example,
the HM51W4160AL L version has a refresh cycle of 1024 cycles/128 ms compared with 1024
cycles/16 ms for the normal version). With these DRAMs, however, the same refresh cycle as
for the normal version is requested only in the case of refreshing immediately following self-
refreshing. To ensure efficient DRAM refreshing, therefore, processing is needed to generate
an overflow interrupt and restore the refresh cycle to the proper value, after CAS-before-RAS
refreshing has been performed following self-refreshing of an L-version DRAM, using the
OVF, OVIE, and LMTS bits in RTCSR and the refresh controller’s refresh count register
(RFCR). The necessary procedure is as follows.
Rev. 3.0, 04/02, page 390 of 1064