Datasheet

Section 6 Bus Controller
REJ09B0140-0900 Rev. 9.00 Page 147 of 846
Sep 16, 2010
H8S/2215 Group
6.10 Bus Arbitration
This LSI has a bus arbiter that arbitrates bus master operations.
There are three bus masters, the CPU, DMAC, and DTC, which perform read/write operations
when they have possession of the bus. Each bus master requests the bus by means of a bus request
signal. The bus arbiter determines priorities at the prescribed timing, and permits use of the bus by
means of a bus request acknowledge signal. The selected bus master then takes possession of the
bus and begins its operation.
6.10.1 Operation
The bus arbiter detects the bus masters’ bus request signals, and if the bus is requested, sends a bus
request acknowledge signal to the bus master making the request. If there are bus requests from
more than one bus master, the bus request acknowledge signal is sent to the one with the highest
priority. When a bus master receives the bus request acknowledge signal, it takes possession of the
bus until that signal is canceled.
The order of priority of the bus masters is as follows:
(High) DMAC > DTC > CPU (Low)
An internal bus access by an internal bus master, and external bus release, can be executed in
parallel.
In the event of simultaneous external bus release request, and internal bus master external access
request generation, the order of priority is as follows:
(High) External bus release > Internal bus master external access (Low)
6.10.2 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 DMAC
and 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: