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Section 12 Bus State Controller (BSC)
Rev. 4.00 Sep. 14, 2005 Page 399 of 982
REJ09B0023-0400
12.5.12 Bus Arbitration
The bus arbitration of this LSI has the bus mastership in the normal state and releases the bus
mastership after receiving a bus request from another device.
Bus mastership is transferred at the boundary of bus cycles. Namely, bus mastership is released
immediately after receiving a bus request when a bus cycle is not being performed. The release of
bus mastership is delayed until the bus cycle is complete when a bus cycle is in progress. Even
when from outside the LSI it looks like a bus cycle is not being performed, a bus cycle may be
performing internally, started by inserting wait cycles between access cycles. Therefore, it cannot
be immediately determined whether or not bus mastership has been released by looking at the CSn
signal or other bus control signals. The states that do not allow bus mastership release are shown
below.
1. 16-byte transfer because of a cache miss
2. During write-back operation for the cache
3. Between the read and write cycles of a TAS instruction
4. Multiple bus cycles generated when the data bus width is smaller than the access size (for
example, between bus cycles when longword access is made to a memory with a data bus
width of 8 bits)
5. 16-byte transfer by the DMAC
6. Setting the BLOCK bit in the CMNCR register to 1
The LSI has the bus mastership until a bus request is received from another device. Upon
acknowledging the assertion (low level) of the external bus request signal BREQ, the LSI releases
the bus at the completion of the current bus cycle and asserts the BACK signal. After the LSI
acknowledges the negation (high level) of the BREQ signal that indicates the external device has
released the bus, it negates the BACK signal and resumes the bus usage.
The SDRAM interface issues all bank pre-charge commands (PALLs) when active banks exist and
releases the bus after completion of a PALL command.
The bus sequence is as follows. The address bus and data bus are placed in a high-impedance state
synchronized with the rising edge of CKIO. The bus mastership enable signal is asserted 0.5
cycles after the above timing, synchronized with the falling edge of CKIO. The bus control signals
(BS, CSn, RASU, RASL, CASU, CASL, CKE, DQMxx, WEn, RD, and RD/WR) are placed in
the high-impedance state at subsequent rising edges of CKIO. Bus request signals are sampled at
the falling edge of CKIO. Even when the bus is released, signals CKE, RASU, RASL, CASU, and
CASL can be driven with previous values according to the setting of the HIZCNT bit in CMNCR.