Renesas HD6417641, Hardware Manual, Page: 408 / 1036

Section 12 Bus State Controller (BSC)
Rev. 4.00 Sep. 14, 2005 Page 358 of 982
REJ09B0023-0400
Bank Active: The synchronous DRAM bank function is used to support high-speed accesses to
the same row address. When the BACTV bit in SDCR is 1, accesses are performed using
commands without auto-precharge (READ or WRIT). This function is called bank-active function.
This function is valid only for either the upper or lower bits of area 3. When area 3 is set to bank-
active mode, area 2 should be set to normal space. When areas 2 and 3 are both set to SDRAM or
both the upper and lower bits of area 3 are connected to SDRAM, auto pre-charge mode must be
set. In this case, precharging is not performed when the access ends. When accessing the same row
address in the same bank, it is possible to issue the READ or WRIT command immediately,
without issuing an ACTV command. As synchronous DRAM is internally divided into several
banks, it is possible to activate one row address in each bank. If the next access is to a different
row address, a PRE command is first issued to precharge the relevant bank, then when precharging
is completed, the access is performed by issuing an ACTV command followed by a READ or
WRIT command. If this is followed by an access to a different row address, the access time will
be longer because of the precharging performed after the access request is issued. The number of
cycles between issuance of the PRE command and the ACTV command is determined by the
WTRP1 and WTPR0 bits in CS3WCR.
In a write, when an auto-precharge is performed, a command cannot be issued to the same bank
for a period of Trwl + Tap cycles after issuance of the WRITA command. When bank active mode
is used, READ or WRIT commands can be issued successively if the row address is the same. The
number of cycles can thus be reduced by Trwl + Tap cycles for each write.
There is a limit on tRAS, the time for placing each bank in the active state. If there is no guarantee
that there will not be a cache hit and another row address will be accessed within the period in
which this value is maintained by program execution, it is necessary to set auto-refresh and set the
refresh cycle to no more than the maximum value of tRAS.
A burst read cycle without auto-precharge is shown in figure 12.23, a burst read cycle for the same
row address in figure 12.24, and a burst read cycle for different row addresses in figure 12.25.
Similarly, a burst write cycle without auto-precharge is shown in figure 12.26, a burst write cycle
for the same row address in figure 12.27, and a burst write cycle for different row addresses in
figure 12.28.
In figure 12.24, a Tnop cycle in which no operation is performed is inserted before the Tc cycle
that issues the READ command. The Tnop cycle is inserted to acquire two cycles of CAS latency
for the DQMxx signal that specifies the read byte in the data read from the SDRAM. If the CAS
latency is specified as two cycles or more, the Tnop cycle is not inserted because the two cycles of
latency can be acquired even if the DQMxx signal is asserted after the Tc cycle.