11242 datasheet(90/441 Pages) FREESCALE

90 / 441 page

MOTOROLA

MC68340 USER’S MANUAL

3- 41

Figure 3-22 is a flowchart showing bus arbitration for a single device. This technique

allows processing of bus requests during data transfer cycles. Refer to Figures 3-23 and

3-24 for bus arbitration timing diagrams.

BR is negated at the time that BGACK is asserted. This type of operation applies to a

system consisting of the MC68340 and one device capable of bus mastership. In a system

having a number of devices capable of bus mastership,

BR from each device can be wire-

ORed to the MC68340. In such a system, more than one bus request could be asserted

simultaneously.

BG is negated a few clock cycles after the transition of BGACK. However,

if bus requests are still pending after the negation of

BG, the MC68340 asserts another BG

within a few clock cycles after it was negated. This additional assertion of

BG allows

external arbitration circuitry to select the next bus master before the current bus master

has finished using the bus. The following paragraphs provide additional information about

the three steps in the arbitration process. Bus arbitration requests are recognized during

normal processing,

HALT assertion, and a CPU32 halt caused by a double bus fault.

GRANT BUS ARBITRATION

1. ASSERT BG

TERMINATE ARBITRATION

1. NEGATE BG (AND WAIT FOR

BGACK TO BE NEGATED)

RE-ARBITRATE OR RESUME

PROCESSOR OPERATION

PROCESSOR

REQUESTING DEVICE

REQUEST THE BUS

1. ASSERT BR

ACKNOWLEDGE BUS MASTERSHIP

1. EXTERNAL ARBITRATION DETERMINES

NEXT BUS MASTER

2. NEXT BUS MASTER WAITS FOR BGACK

TO BE NEGATED

3. NEXT BUS MASTER ASSERTS BGACK

TO BECOME NEW MASTER

4. BUS MASTER NEGATES BR

OPERATE AS BUS MASTER

RELEASE BUS MASTERSHIP

1. NEGATE BGACK

1. PERFORM DATA TRANSFERS (READ AND

WRITE CYCLES) ACCORDING TO THE

SAME RULES THE PROCESSOR USES

Figure 3-22. Bus Arbitration Flowchart for Single Request

Freescale Semiconductor, Inc.

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11242 Datasheet(PDF) 90 Page - Freescale Semiconductor, Inc

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