SH7280 Datasheet, PDF (362/1726 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperHTM RISC engine family

Section 9 Bus State Controller (BSC)
(3) Burst Read
A burst read occurs in the following cases with this LSI.
• Access size in reading is larger than data bus width.
• 16-byte transfer in DMAC
This LSI always accesses the SDRAM with burst length 1. For example, read access of burst
length 1 is performed consecutively 8 times to read 16-byte continuous data from the SDRAM that
is connected to a 16-bit data bus. This access is called the burst read with the burst number 8.
Table 9.14 shows the relationship between the access size and the number of bursts.
Table 9.14 Relationship between Access Size and Number of Bursts
Bus Width
16 bits
Access Size
8 bits
16 bits
32 bits
16 bytes
Number of Bursts
1
1
2
8
Figures 9.17 and 9.18 show a timing chart in burst read. In burst read, an ACTV command is
output in the Tr cycle, the READ command is issued in the Tc1, Tc2, and Tc3 cycles, the READA
command is issued in the Tc4 cycle, and the read data is received at the rising edge of the external
clock (CK) in the Td1 to Td4 cycles. The Tap cycle is used to wait for the completion of an auto-
precharge induced by the READA command in the SDRAM. In the Tap cycle, a new command
will not be issued to the same bank. However, access to another CS space or another bank in the
same SDRAM space is enabled. The number of Tap cycles is specified by the WTRP1 and
WTRP0 bits in CS3WCR.
In this LSI, wait cycles can be inserted by specifying each bit in CS3WCR to connect the SDRAM
in variable frequencies. Figure 9.18 shows an example in which wait cycles are inserted. The
number of cycles from the Tr cycle where the ACTV command is output to the Tc1 cycle where
the READ command is output can be specified using the WTRCD1 and WTRCD0 bits in
CS3WCR. If the WTRCD1 and WTRCD0 bits specify one cycles or more, a Trw cycle where the
NOT command is issued is inserted between the Tr cycle and Tc1 cycle. The number of cycles
from the Tc1 cycle where the READ command is output to the Td1 cycle where the read data is
latched can be specified for the CS2 and CS3 spaces independently, using the A2CL1 and A2CL0
bits in CS2WCR or the A3CL1 and A3CL0 bits in CS3WCR and WTRCD0 bit in CS3WCR. The
number of cycles from Tc1 to Td1 corresponds to the SDRAM CAS latency. The CAS latency for
the SDRAM is normally defined as up to three cycles. However, the CAS latency in this LSI can
Rev. 1.00 Jun. 26, 2008 Page 332 of 1692
REJ09B0393-0100