Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 400

External Bus Interface
arbiter may take away the BG if an internal master of higher priority requests the bus and the new master
does not assert BB within one clock after BG.)
If the arbiter removes the bus grant from a device that wants another transfer, the device must rearbitrate
for bus mastership. The MPC885, however, guarantees data coherency for accesses to small ports and for
decomposed bursts. This means that the MPC885 does not release the bus before atomic transactions
complete. For example, a half-word transfer to a byte port is broken into two byte transfers; the MPC885
does not deassert BB until the second transfer completes, unless an error occurs.
bus arbitration protocol.
prioritization can be configured.
Requesting Device
Assert BR
1. Wait for BB to be negated
2. Assert BB to become next master
3. Negate BR
Operate as bus master
Perform data transfer
Release bus mastership
Negate BB
13.4.6.1 Bus Request (BR)
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 requests are pending, the master can
assert BR as long as needed. When configured for external arbitration, the MPC885 drives BR when it
requires bus mastership. When the internal on-chip arbiter is used, BR is an input to the internal arbiter
and should be driven by the external bus master.
13.4.6.2 Bus Grant (BG)
The arbiter asserts BG to indicate that the bus is granted to the requesting device. BG can be negated after
BR is negated or it can remain asserted to park the current master on the bus. The internal arbiter may take
away the BG if the new master does not assert BB within one clock.
13-26
Section 10.4.2, "SIU Module Configuration Register (SIUMCR),"
Request the bus
Request the bus
Figure 13-21. Basic Bus Arbitration Protocol
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Figure 13-21
Arbiter
Grant bus
Assert BG
Terminate arbitration
Negate BG (or keep it asserted to park
bus master)
shows basic
describes how
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