Renesas SH7781 User Manual
Page 544
12. DDR2-SDRAM Interface (DBSC2)
Rev.1.00 Jan. 10, 2008 Page 514 of 1658
REJ09B0261-0100
16-bit external bus
Read (16 bytes)
Read (1, 2, 4, 8, or 16 bytes)
Read (32 bytes)
Write (1, 2, 4, 8, or 16 bytes)
Write (32 bytes)
Read (1, 2, 4, or 8 bytes)
Read (32 bytes)
Write (1, 2, 4, or 8 bytes)
Write (16 bytes)
Read
Write (32 bytes)
Write
Read
Read
Write
Write
Read
Read
Read
Read
Write
Write
Write
Write
DSEL
DSEL
DSEL
DSEL
DSEL
DSEL
DSEL
DSEL
Read
Read
Read
DSEL
Write
Write
Write
DSEL
32-bit external bus
1 command
2 commands
4 commands
1 command
2 commands
4 commands
1 command
2 commands
1 command
2 commands
1st cycle
2nd cycle
3rd cycle
4th cycle
5th cycle
6th cycle
7th cycle
Figure 12.5 Read/Write Command Issued to the SDRAM in Response
to the Request from the SuperHyway Bus
(2)
Preceding Precharge/Activate Processing
In order to utilize DDR2-SDRAM multibank functions to reduce SDRAM command vacant cycles
insofar as possible and improve the efficiency of bus use, the DBSC2 issues in advance a
PRE/ACT command corresponding to the following request queue page miss processing. Only the
PRE/ACT command is issued in advance, so there is no change in the read/write order. A
PRE/ACT command is issued in advance only when the following request (1) results in a page
miss, and moreover (2) entails access of a bank different from that of the request currently being
processed. Figure 12.6 shows an example of execution of preceding precharge/activate processing.
This is an example of a command issued to the SDRAM when the external data bus width is 32
bits, the PRE/ACT minimum time constraint is 3 cycles, the ACT-READ/WRITE minimum time
constraint is 3 cycles, and the ACT (A)-ACT (B) minimum time constraint is 2 cycles. In this
example, the first through fourth requests are accumulated, and the first request is the request
initially provided to the queue.
First, at time 1 the DBSC2 issues to the SDRAM a PRE command for the first read (16-byte)
request processing. Then, when determining the command to be issued at time 2, due to timing
constraints it is not possible to issue at time 2 the ACT command necessary as request processing
for the first read (16-byte) request, which has higher priority. Hence the DBSC2 searches for a