Bus Request (Br); Bus Grant (Bg) - Freescale Semiconductor MPC850 User Manual

Requesting Device
Request the bus
Assert BR
Request the bus
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 MPC850 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.
When the internal on-chip arbiter is used, BG is an output from the internal arbiter to the
external bus master. When configured for external central arbitration, BG is an input to the
MPC850 from the external arbiter.
Figure 13-21. Bus Arbitration Flowchart
Chapter 13. External Bus Interface
Arbiter
Grant bus
Assert BG
Terminate arbitration
Negate BG (or keep it asserted to
park bus master)
Bus Operations