145
4.5.9
Precharge State Control
When DRAM is accessed, a
RAS
precharge time must be secured. With the H8S/2678 Series, one
T
p
state is always inserted when DRAM space is accessed. From one to four T
p
states can be
selected by setting bits TPC1 and TPC0 in DRACCR. Set the optimum number of T
p
cycles
according to the DRAM connected and the operating frequency of the chip. Figure 4.24 shows the
timing when two Tp states are inserted.
The setting of bits TPC1 and TPC0 is also valid for T
p
states in refresh cycles.
T
p1
ø
RASn
(
CSn
)
Read
Write
UCAS
,
LCAS
WE
(
HWR
)
OE
(
RD
)
Data bus
WE
(
HWR
)
OE
(
RD
)
Data bus
Address bus
T
p2
T
r
T
c1
T
c2
Row address
Column address
High
High
Note: n = 2 to 5
Figure 4.24 Example of Timing with Two-State Precharge Cycle
(RAST = 0, CAST = 0)
Содержание H8S/2670
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