Bus Arbitration; Overview; Operation; Bus Transfer Timing - Hitachi H8S/2646 Hardware Manual

Hitachi 16-bit single-chip microcomputer h8s/2646 series

Table of Contents

Bus Arbitration

The H8S/2646 Series has a bus arbiter that arbitrates bus master operations.

There are two bus masters, the CPU 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.

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:

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.