156
When the clock is selected with the CKS2–CKS0 bits, RTCNT immediately begins to increment
from its current value. This means that when the RTCOR cycle is set after the CKS2–CKS0 bits
are set, the RTCNT count may already be higher than the RTCOR cycle. When this occurs, the
RTCNT will overflow once (from H'FF to H'00) and incrementing will start again. Since the CBR
refresh will not be performed until the RTCNT again matches the RTCOR value, the initial refresh
interval will be rather long. It is thus advisable to set the RTCOR cycle prior to setting the CKS2–
CKS0 bits and start it incrementing. When CBR refresh control is being performed after use as an
8-bit interval timer, the RTCNT count value may be in excess of the refresh cycle. For this reason,
clear RTCNT by writing H'00 before starting refresh control to assure a correct refresh interval.
When the RW1 bit in WCR1 is set to 1 and the read cycle is set to long pitch, the number of wait
states selected by the RLW1 and RLW0 bits in RCR will be inserted into the CBR refresh cycle,
regardless of the status of the
WAIT
signal. Figure 8.29 shows RTCNT operation and figure 8.30
shows the timing of the CBR refresh. For details on timing, see sections 20.1.3 (3) and 20.2.3 (3),
Bus Timing.
CBR
CBR
CBR
CBR
Clock
selected with
CKS2–CKS0
H'00
RTCOR
value
RTCNT
value
Compare
match
with RTCOR
Compare
match
with RTCOR
Compare
match
with RTCOR
Compare
match
with RTCOR
CBR: CAS-before-RAS refresh
Time
Figure 8.29 Refresh Timer Counter (RTCNT) Operation
T
Rp
T
Rr
T
Rc
CK
RAS
CAS
Figure 8.30 Output Timing for CAS-Before-RAS Refresh Signal
Содержание HD6417032
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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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