CHAPTER 6 BUS CONTROL
User’s Manual U14272EJ3V0UM
115
6.2.4 BCURFCNTREG (0x0A00 0010)
Bit
15
14
13
12
11
10
9
8
Name
Reserved
Reserved
BRF13
BRF12
BRF11
BRF10
BRF9
BRF8
R/W
R
R
R/W
R/W
R/W
R/W
R/W
R/W
At reset
0
0
0
1
1
1
1
1
Bit
7
6
5
4
3
2
1
0
Name
BRF7
BRF6
BRF5
BRF4
BRF3
BRF2
BRF1
BRF0
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
At reset
1
1
1
1
1
1
1
1
Bit
Name
Function
15, 14
Reserved
0 is returned when read
13 to 0
BRF(13:0)
These bits select the DRAM refresh rate that is based on the TClock. The refresh
rate is obtained by following expression.
Refresh rate = BRF(13:0) x TClock period
For example, to select a 15.6
µ
s refresh rate with a 50 MHz TClock:
BRF(13:0) = 15600 (ns) / 20 (ns) = 0x30C
Remarks 1. When the IORDY signal does not become high level though the DRAM refresh rate has elapsed
during the external ISA memory or I/O cycles, a DRAM refresh cycle may be lost.
2. Refresh timing is generated from detecting match between values of the internal up counter and
BCURFCNTREG register. Therefore, when the BCURFCNTREG register value is changed smaller
than current value, and if the internal counter value is larger than the new BCURFCNTREG register
value, the next CBR refresh timing is at next match after the counter rounds over.
Содержание VR4181 mPD30181
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