HD6432351 datasheet(222/1025 Pages) RENESAS

222 / 1025 page

Section 6 Bus Controller

Rev. 3.00 Sep 15, 2006 page 188 of 988

REJ09B0330-0300

6.10

Bus Release

6.10.1

Overview

The H8S/2350 Group can release the external bus in response to a bus request from an external

device. In the external bus released state, the internal bus master continues to operate as long as

there is no external access.

If an internal bus master wants to make an external access in the external bus released state, or if a

refresh request is generated, it can issue a bus request off-chip.

6.10.2

Operation

In external expansion mode, the bus can be released to an external device by setting the BRLE bit

in BCRL to 1. Driving the

BREQ pin low issues an external bus request to the H8S/2350 Group.

When the

BREQ pin is sampled, at the prescribed timing the BACK pin is driven low, and the

address bus, data bus, and bus control signals are placed in the high-impedance state, establishing

the external bus-released state.

In the external bus released state, an internal bus master can perform accesses using the internal

bus. When an internal bus master wants to make an external access, it temporarily defers

activation of the bus cycle, and waits for the bus request from the external bus master to be

dropped. Even if a refresh request is generated in the external bus released state, refresh control is

deferred until the external bus master drops the bus request.

If the BREQOE bit in BCRL is set to 1, when an internal bus master wants to make an external

access in the external bus released state, or when a refresh request is generated, the

BREQO pin is

driven low and a request can be made off-chip to drop the bus request.

When the

BREQ pin is driven high, the BACK pin is driven high at the prescribed timing and the

external bus released state is terminated.

If an external bus release request and external access occur simultaneously, the order of priority is

as follows:

(High) External bus release > Internal bus master external access (Low)

If a refresh request and external bus release request occur simultaneously, the order of priority is

as follows:

(High) Refresh > External bus release (Low)

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