Datasheet

Section 6 Bus Controller
Rev.7.00 Feb. 14, 2007 page 181 of 1108
REJ09B0089-0700
6.8.3 Bus Transfer Timing
Even if a bus request is received from a bus master with a higher priority than that of the bus
master that has acquired the bus and is currently operating, the bus is not necessarily transferred
immediately. There are specific times at which each bus master can relinquish the bus.
CPU: The CPU is the lowest-priority bus master, and if a bus request is received from the DTC,
the bus arbiter transfers the bus to the bus master that issued the request. The timing for transfer of
the bus is as follows:
• The bus is transferred at a break between bus cycles. However, if a bus cycle is executed in
discrete operations, as in the case of a longword-size access, the bus is not transferred between
the operations. See appendix A.5, Bus States during Instruction Execution, for timings at
which the bus is not transferred.
• If the CPU is in sleep mode, it transfers the bus immediately.
DTC: The DTC sends the bus arbiter a request for the bus when an activation request is generated.
The DTC can release the bus after a vector read, a register information read (3 states), a single data
transfer, or a register information write (3 states). It does not release the bus during a register
information read (3 states), a single data transfer, or a register information write (3 states).
6.8.4 External Bus Release Usage Note
External bus release can be performed on completion of an external bus cycle. The RD signal
remains low until the end of the external bus cycle. Therefore, when external bus release is
performed, the RD signal may change from the low level to the high-impedance state.
6.9 Resets and the Bus Controller
In a reset, the chip, including the bus controller, enters the reset state at that point, and any
executing bus cycle is discontinued.