155
•
RAS up mode: When the RASD bit is cleared to 0, the
RAS
signal reverts to high whenever a
DRAM access pauses for access to another space. Burst operation continues only while
DRAM access is continuous. Figure 8.28 shows the timing when an external memory space
access occurs during burst operation in RAS up mode.
T
p
T
r
T
c
T
c
CK
A21–
A0
RAS
CAS
Data 1
AD15–
AD0
T1
DRAM access
DRAM access
External memory
space access
T
c
T
r
Data 2
Data 3
T
p
Row address
Row address
External
memory data
Column
address 1
Column
address 2
External memory
address
Column
address 3
Figure 8.28 RAS Up Mode
8.5.6
Refresh Control
The BSC has a function for controlling DRAM refreshing. By setting the refresh mode bit
(RMODE) in the refresh control register (RCR), either CAS-before-RAS refresh (CBR) or self-
refresh can be selected. When no refresh is performed, the refresh timer counter (RTCNT) can be
used as an 8-bit interval timer.
CAS-Before-RAS Refresh (CBR): A refresh is performed at an interval determined by the input
clock selected with clock select bits 2–0 (CKS2–CKS0) in the refresh timer control/status register
(RTCSR) and the value set in the refresh time constant register (RTCOR). Set the values of
RTCOR and CKS2–CKS0 so they satisfy the refresh interval specifications of the DRAM being
used.
To perform a CBR refresh, clear the RMODE bit in RCR to 0 and then set the refresh control bit
(RFSHE) bit to 1. Also write the required values to RTCNT and RTCOR. When the clock is
subsequently selected with the CKS2–CKS0 bits in RTCSR, RTCNT will begin to increment from
its current value. The RTCNT value is constantly compared with the RTCOR value and a CBR
refresh is performed when they match. RTCNT is simultaneously cleared to H'00 and
incrementing begins again.
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Страница 525: ...490 CK RAS CAS TRp TRc TRcc tRASD1 tRASD2 tCASD3 tCASD2 TRr tCSR Figure 20 18 Self Refresh ...
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