HD6417021 Hitachi Semiconductor, HD6417021 Datasheet - Page 158

• Manufacturer Part Number: HD6417021
• Description: SuperH RISC engine
• Manufacturer: Hitachi Semiconductor

8.5.5
In addition to the normal mode of DRAM access, in which row addresses are output at every
access and data then accessed (full access), the DRAM also has a high-speed page mode for use
when continuously accessing the same row. The high speed page mode enables fast access of data
simply by changing the column address after the row address is output (burst mode). Select
between full access and burst operation by setting the burst enable bit (BE)) in the DCR. When the
BE bit is set to 1, burst operation is performed when the row address matches the previous DRAM
access row address. Figure 8.22 shows the comparison of full access and burst operation.
Short pitch high-speed page mode or long pitch high-speed page mode burst transfers can be
selected independently for DRAM read/write cycles even when the burst operation is selected by
using the bits corresponding to area 1 in WCR1 and WCR2 (RW1, WW1, DRW1, DWW1). The
RAS down mode or RAS up mode can be selected by setting the RAS down bit (RASD) of the
DCR when there is an access outside the DRAM space during burst operation.
142 HITACHI
A21–A0
A21–A0
AD15–
AD15–
RAS
CAS
RAS
CAS
AD0
AD0
DRAM Burst Mode
Row address 1
Figure 8.22 Full Access and Burst Operation
Row address 1
(b) Burst operation (read cycle)
address 1
(a) Full access (read cycle)
Column
address 2
Column address 1
Data 1
Data 1
Column
Row address 2
address 3
Data 2
Column
address 4
Data 3
Column
Data 2
Data 4
address 2
Column