SH7721 datasheet(436/1524 Pages) RENESAS

436 / 1524 page

Section 9

Bus State Controller (BSC)

Rev. 3.00 Jan. 18, 2008 Page 374 of 1458

REJ09B0033-0300

(8)

Refreshing

This LSI has a function for controlling SDRAM refreshing. Auto-refreshing can be performed by

clearing the RMODE bit to 0 and setting the RFSH bit to 1 in SDCR. A continuous refreshing can

be performed by setting the RRC[2:0] bits in RTCSR. If SDRAM is not accessed for a long

period, self-refresh mode, in which the power consumption for data retention is low, can be

activated by setting both the RMODE bit and the RFSH bit to 1.

(a)

Auto-refreshing

Refreshing is performed at intervals determined by the input clock selected by bits CKS[2:0] in

RTCSR, and the value set by in RTCOR. The value of bits CKS[2:0] in RTCOR should be set so

as to satisfy the refresh interval stipulation for the SDRAM used. First make the settings for

RTCOR, RTCNT, and the RMODE and RFSH bits in SDCR, then make the CKS[2:0] and

RRC[2:0] settings. When the clock is selected by bits CKS[2:0], RTCNT starts counting up from

the value at that time. The RTCNT value is constantly compared with the RTCOR value, and if

the two values are the same, a refresh request is generated and an auto-refresh is performed for the

number of times specified by the RRC[2:0]. At the same time, RTCNT is cleared to 0 and the

count-up is restarted. Figure 9.25 shows the auto-refresh cycle timing.

After starting, the auto refreshing, PALL command is issued in the Tp cycle to make all the banks

to precharged state from active state when some bank is being precharged. Then REF command is

issued in the Trr cycle after inserting idle cycles of which number is specified by the TRP[1:0]bits

in CSnWCR. A new command is not issued for the duration of the number of cycles specified by

the TRC[1:0] bits in CSnWCR after the Trr cycle. The TRC[1:0] bits must be set so as to satisfy

the SDRAM refreshing cycle time stipulation (tRC). A NOP cycle is inserted between the Tp

cycle and Trr cycle when the setting value of the TRP[1:0] bits in CSnWCR is longer than or

equal to 2 cycles.

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