MC68340 datasheet(92/441 Pages) FREESCALE

92 / 441 page

MOTOROLA

MC68340 USER’S MANUAL

3- 43

3.6.1 Bus Request

External devices capable of becoming bus masters request the bus by asserting

BR. This

signal can be wire-ORed to indicate to the MC68340 that some external device requires

control of the bus. The MC68340 is effectively at a lower bus priority level than the

external device and relinquishes the bus after it has completed the current bus cycle (if

one has started). If no

BGACK is received while the BR is active, the MC68340 remains

bus master once

BR is negated. This prevents unnecessary interference with ordinary

processing if the arbitration circuitry inadvertently responds to noise or if an external

device determines that it no longer requires use of the bus before it has been granted

mastership.

3.6.2 Bus Grant

The MC68340 supports operand coherency; thus, if an operand transfer requires multiple

bus cycles, the MC68340 does not release the bus until the entire transfer is complete.

Therefore, assertion of

BG is subject to the following constraints:

• The minimum time for

BG assertion after BR is asserted depends on internal

synchronization (see Section 11 Electrical Characteristics).

• During an external operand transfer, the MC68340 does not assert

BG until after

the last cycle of the transfer (determined by SIZx and

DSACK

≈).

• During an external operand transfer, the MC68340 does not assert

BG as long as

RMC is asserted.

• If the show cycle bits SHEN1–SHEN0 = 01, the MC68340 does not assert

BG to

an external master.

Externally, the

BG signal can be routed through a daisy-chained network or a priority-

encoded network. The MC68340 is not affected by the method of arbitration as long as the

protocol is obeyed.

3.6.3 Bus Grant Acknowledge

An external device cannot request and be granted the external bus while another device is

the active bus master. A device that asserts

BGACK remains the bus master until it

negates

BGACK. BGACK should not be negated until all required bus cycles are

completed. Bus mastership is terminated at the negation of

BGACK.

Once an external device receives the bus and asserts

BGACK, it should negate BR. If BR

remains asserted after

BGACK is asserted, the MC68340 assumes that another device is

requesting the bus and prepares to issue another

BG.

Freescale Semiconductor, Inc.

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