MPC555
/
MPC556
EXTERNAL BUS INTERFACE
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
9-31
ecute a new cycle, the master must re-arbitrate before a new cycle can be executed.
The MPC555
/ MPC556, however, guarantees data coherency for access to a small
port size and for decomposed bursts. This means that the MPC555
/ MPC556 will not
release the bus before the completion of the transactions that are considered atomic.
describes the basic protocol for bus arbitration.
Figure 9-23 Bus Arbitration Flowchart
9.5.6.1 Bus Request
The potential bus master asserts BR to request bus mastership. BR should be negated
as soon as the bus is granted, the bus is not busy, and the new master can drive the
bus. If more requests are pending, the master can keep asserting its bus request as
long as needed. When configured for external central arbitration, the MPC555
/
MPC556 drives this signal when it requires bus mastership. When the internal on-chip
arbiter is used, this signal is an input to the internal arbiter and should be driven by the
external bus master.
Requesting Device
Arbiter
Request the Bus
1. Assert BR
Terminate Arbitration
1. Negate BG (or keep asserted to park
1. Wait for BB to be negated.
3. Negate BR
bus master)
Operate as Bus Master
1. Perform data transfer
Release Bus Mastership
1. Negate BB
Acknowledge Bus Mastership
2. Assert BB to become next master
Grant Bus Arbitration
1. Assert BG
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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