SH7065 Datasheet, PDF (305/941 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperH RISC engine Family/SH7000 Series

Section 8 Bus State Controller (BSC)
CKE
CSn
TRc
TRr1
TRrw
TRr2
RDWR
RASn
CASxxn
Figure 8.29 Basic Timing of DRAM CAS-Before-RAS Refresh Cycle
Self-Refreshing: The self-refreshing supported by the SH7065 is shown in figure 8.30.
A transition to self-refresh mode is made by setting the RFSH bit and RMD bit to 1 in DCR. Self-
refresh mode is exited by clearing the RMD bit to 0 in DCR, than performing CAS-before-RAS
refreshing on all row addresses within the time specified for the DRAM. The RAS precharge time
immediately after the end of self-refreshing can be set with the TPCS bits in DCR. If there is a
delay between clearing self-refreshing and the start of CAS-before-RAS refreshing, this must be
taken into consideration when setting the initial RTCNT value. When the RTCNT value is set to
the same value as RTCOR, a refresh request is issued immediately.
To protect DRAM data, the DRAM should not be accessed during self-refreshing. If DRAM is to
be accessed during self-refreshing, first clear self-refreshing, then perform refreshing of all row
addresses before making the access.
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 the necessary 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. 5.00 Sep 11, 2006 page 283 of 916
REJ09B0332-0500