Bus Arbitration; Operation; Bus Transfer Timing - Hitachi H8S/2378, H8S/2378R Series Hardware Manual

6.12

Bus Arbitration

This LSI has a bus arbiter that arbitrates bus master operations (bus arbitration).
There are four bus masters—the CPU, DTC, DMAC, and EXDMAC—that 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.
6.12.1

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. 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) EXDMAC > DMAC > DTC > CPU (Low)
An internal bus access by internal bus masters except the EXDMAC and external bus release, a
refresh when the CBRM bit is 0, and an external bus access by the EXDMAC can be executed in
parallel.
If an external bus release request, a refresh request, and an external access by an internal bus
master occur simultaneously, the order of priority is as follows:
(High) Refresh > EXDMAC > External bus release (Low)
(High) External bus release > External access by internal bus master except EXDMAC (Low)
As a refresh when the CBRM bit in REFCR is cleared to 0 and an external access other than to
DRAM space by an internal bus master can be executed simultaneously, there is no relative order
of priority for these two operations.
6.12.2

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 timings at which each bus master can relinquish the bus.
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