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MPC563XM Reference Manual, Rev. 1
364
Freescale Semiconductor
Preliminary—Subject to Change Without Notice
By default, internal and external masters are treated with equal priority, with each having to relinquish the
bus after the current transaction if another master is requesting it. If internal and external requests for the
bus occur in the same cycle, the internal arbiter grants the bus to the master who least recently used the
bus. If no other master is requesting the bus, the bus continues to be granted to the current master, and the
current master may start another access without rearbitrating for the bus.
If the priority field is configured for unequal priority between internal and external masters, then whenever
requests are pending from both masters, the one with higher priority is always granted the bus. However,
in all cases, a transaction in progress (or that has already been granted, e.g. MCU Bus Wait and Ext. Bus
Wait states) is allowed to complete, even when a request from a higher priority master is pending.
There is a minimum of one cycle between the posedge CLKOUT that a BR assertion is sampled by the
EBI and the posedge CLKOUT where BG is driven out asserted by the EBI. This is to avoid timing
problems that would otherwise limit the frequency of operation in External Master Mode.
The external master is given 2 cycles to start its access after a posedge CLKOUT in which bus grant was
given to it by the internal arbiter (BG asserted, BB negated for 2 cycles). This means when BG is negated
(to take away bus grant from the external master), the EBI does not start an access of its own for 3 cycles
(1 extra cycle in order to detect external BB assertion). If the external master jumps on the bus (by asserting
BB) during the 2-cycle window, the EBI detects the BB assertion and delays starting its access until the
external master access has completed (BB negated for 2 cycles).
shows this 2-cycle window
of opportunity.
I
Figure 13-35. Internal Arbitration 2-Cycle Window-of-Opportunity)
shows example timing for the case of one master using internal arbitration (Master 0), while
another master is configured for external arbitration (Master 1). In this case, the BR signals of each master
CLKOUT
BR
BG
BB (case 1)
Using Internal arbiter for Master 0,
M1 receives bus grant and
earliest cycle M1 can assert BB
BB (case 2)
latest cycle M1 can assert BB
window-of-opportunity
* earliest cycle M0 can assert BB if M1 has not asserted BB yet
*
external arbitration for Master 1
BB negated for 2nd cycle