Motorola MC68340 User Manual

MOTOROLA

3- 43

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.

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.

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

.

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Freescale Semiconductor, Inc.

For More Information On This Product,

   Go to: www.freescale.com

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