SH7708 datasheet(269/633 Pages) RENESAS

269 / 633 page

251

Refresh Timing: The bus state controller includes a function for controlling DRAM refreshing.

Refreshing using a CAS-before-RAS cycle can be performed by clearing the RMODE bit to 0 and

setting the RFSH bit to 1 in MCR for area 3 DRAM, or by clearing the RMODE bit to 0 and

setting the RFSH bit to 1 in DCR for area 2 DRAM. It also supports self-refresh mode.

When CAS-before-RAS refresh cycles are executed, refreshing is performed at intervals

determined by the input clock selected by bits CKS2–CKS0 in RTCSR, and the value set in

RTCOR. The value of bits CKS2–CKS0 in RTCOR should be set so as to satisfy the stipulation

for the DRAM refresh interval. First make the settings for RTCOR, RTCNT, and the RMODE and

RFSH bits in MCR, then make the CKS2–CKS0 setting. When the clock is selected by CKS2 to

CKS0, 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 the

IRQOUT pin goes low. If the SH7708 Series’ external bus can be used, CAS-

before-RAS refreshing is performed, and if there is no other interrupt request the

IRQOUT pin

goes high. At the same time, RTCNT is cleared to zero and the count-up is restarted. Figure 10.20

shows the operation of CAS-before-RAS refreshing.

RTCOR value

RTCNT

H'00000000

RTCSR.CKS(2–0)

CMF

External bus

CMF flag cleared by start of

refresh cycle

= 000

≠ 000

RTCNT cleared to 0 when

RTCNT = RTCOR

CAS-before-RAS refresh cycle

Time

Figure 10.20 CAS-Before-RAS Refresh Operation

SH7708 데이터시트(PDF) 269 Page - Renesas Technology Corp