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MC68F375
SINGLE-CHIP INTEGRATION MODULE 2 (SCIM2E)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
4-51
indicating that no other bus master is active. This technique allows the processing of
bus requests during data transfer cycles.
BG is negated a few clock cycles after BGACK transition. However, if bus requests are
still pending after BG is negated, the MCU asserts BG again within a few clock cycles.
This additional BG assertion allows external arbitration circuitry to select the next bus
master before the current master has released the bus.
Refer to Figure 4-16 which shows bus arbitration for a single device. The flow chart
shows BR negated at the same time BGACK is asserted. Refer to the SCIM Refer-
ence Manual (SCIMRM/AD) for more information on bus arbitration.
Figure 4-16 Bus Arbitration Flowchart for Single Request
4.6.6.1 Show Cycles
The MCU normally performs internal data transfers without affecting the external data
bus, but it is possible to show these transfers during debugging. AS is not asserted
externally during show cycles.
Show cycles are controlled by SHEN[1:0] in SCIMMCR. This field set to 0b00 by reset.
When show cycles are disabled, the address bus, function codes, size, and read/write
signals reflect internal bus activity, but AS and DS are not asserted externally and
external data bus pins are in a high-impedance state during internal accesses. Refer
more information.
GRANT BUS ARBITRATION
1) ASSERT BUS GRANT (BG)
TERMINATE ARBITRATION
1) NEGATE BG (AND WAIT FOR
BGACK TO BE NEGATED)
RE-ARBITRATE OR RESUME PROCESSOR
OPERATION
MCU
REQUESTING DEVICE
REQUEST THE BUS
1) ASSERT BUS REQUEST (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
1) PERFORM DATA TRANSFERS (READ AND
WRITE CYCLES) ACCORDING TO THE SAME
RULES THE PROCESSOR USES
RELEASE BUS MASTERSHIP
1) NEGATE BGACK
BUS ARB FLOW
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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