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MOTOROLA
MC68306 USER'S MANUAL
3- 13
There are two ways to arbitrate the bus, 3-wire and 2-wire bus arbitration. The EC000
core can do either 2-wire or 3-wire bus arbitration. Figures 3-12 and 3-14 show 3-wire bus
arbitration and Figures 3-13 and 5-15 show 2-wire bus arbitration.
BGACK
must be pulled
high for 2-wire bus arbitration.
GRANT BUS ARBITRATION
REQUEST THE BUS
1) ASSERT BUS REQUEST (BR)
REQUESTING DEVICE
1) EXTERNAL ARBITRATION DETER-
MINES NEXT BUS MASTER
2) NEXT BUS MASTER WAITS FOR
CURRENT CYCLE TO COMPLETE
3) NEXT BUS MASTER ASSERTS BUS
GRANT ACKNOWLEDGE (BGACK)
TO BECOME NEW MASTER
4) BUS MASTER NEGATES BR
TERMINATE ARBITRATION
1) NEGATE BGACK
PROCESSOR
1) ASSERT BUS GRANT (BG)
ACKNOWLEDGE BUS MASTERSHIP
OPERATE AS BUS MASTER
1) PERFORM DATA TRANSFERS (READ
AND WRITE CYCLES) ACCORDING
TO THE SAME RULES THE PRO-
CESSOR USES
REARBITRATE OR RESUME
PROCESSOR OPERATION
RELEASE BUS MASTERSHIP
1) NEGATE BG (AND WAIT FOR BGACK
TO BE NEGATED)
2) IF BR REMAINS ASSERTED AFTER
BGACK ASSERTED, RE-ASSERT BG.
Figure 3-12. Three-Wire Bus Arbitration Cycle Flowchart