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6.10
Bus Arbiter
The bus controller has a built-in bus arbiter that arbitrates between different bus masters. There
are four bus masters: the CPU, DMA controller (DMAC), DRAM interface, and an external bus
master. When a bus master has the bus right it can carry out read, write, or refresh access. Each
bus master uses a bus request signal to request the bus right. At fixed times the bus arbiter
determines priority and uses a bus acknowledge signal to grant the bus to a bus master, which can
the operate using the bus.
The bus arbiter checks whether the bus request signal from a bus master is active or inactive, and
returns an acknowledge signal to the bus master. When two or more bus masters request the bus,
the highest-priority bus master receives an acknowledge signal. The bus master that receives an
acknowledge signal can continue to use the bus until the acknowledge signal is deactivated.
The bus master priority order is:
(High)
External bus master > DRAM interface > DMAC > CPU
The bus arbiter samples the bus request signals and determines priority at all times, but it does not
always grant the bus immediately, even when it receives a bus request from a bus master with
higher priority than the current bus master. Each bus master has certain times at which it can
release the bus to a higher-priority bus master.
6.10.1
Operation
CPU: The CPU is the lowest-priority bus master. If the DMAC, DRAM interface, or an external
bus master requests the bus while the CPU has the bus right, the bus arbiter transfers the bus right
to the bus master that requested it. The bus right is transferred at the following times:
• The bus right is transferred at the boundary of a bus cycle. If word data is accessed by two
consecutive byte accesses, however, the bus right is not transferred between the two byte
accesses.
• If another bus master requests the bus while the CPU is performing internal operations, such as
executing a multiply or divide instruction, the bus right is transferred immediately. The CPU
continues its internal operations.
• If another bus master requests the bus while the CPU is in sleep mode, the bus right is
transferred immediately.
DMAC: When the DMAC receives an activation request, it requests the bus right from the bus
arbiter. If the DMAC is bus master and the DRAM interface or an external bus master requests
the bus, the bus arbiter transfers the bus right from the DMAC to the bus master that requested the
bus. The bus right is transferred at the following times.
178
(Low)
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