Motorola ColdFire MCF5281 - Manual (Page 161)

System Control Module (SCM)
Freescale Semiconductor 8-9
8.5.2 Arbitration Algorithms
There are two modes of arbitration: fixed and round-robin. This section discusses the differences between
them.
8.5.2.1 Round-Robin Mode
Round-robin arbitration is the default mode after reset. This scheme cycles through the sequence of
masters as specified by MPARK[Mn_PRTY] bits. Upon completion of a transfer, the master is given the
lowest priority and the priority for all other masters is increased by one.
M3 = 11 M2 =01 M1 = 10 M0 = 00
next +1 M3 = 00 M2 =10 M1 = 11 M0 = 01
next +2 M3 = 01 M2 =11 M1 = 00 M0 = 10
next +3 M3 = 10 M2 =00 M1 = 01 M0 = 11
If no masters are requesting, the arbitration unit must “park”, pointing at one of the masters. There are two
possibilities, park the arbitration unit on the last active master, or park pointing to the highest priority
master. Setting MPARK[PRK_LAST] causes the arbitration pointer to be parked on the highest priority
master. In round-robin mode, programming the timeout enable and lockout bits MPARK[13,11:8] will
have no effect on the arbitration.
8.5.2.2 Fixed Mode
In fixed arbitration the master with highest priority (as specified by the MPARK[Mn_PRTY] bits) will win
the bus. That master will relinquish the bus when all transfers to that master are complete.
If MPARK[TIMEOUT] is set, a counter will increment for each master for every cycle it is denied access.
When a counter reaches the limit set by MPARK[LCKOUT_TIME], the arbitration algorithm will be
changed to round-robin arbitration mode until all locks are cleared. The arbitration will then return to fixed
mode and the highest priority master will be granted the bus.
As in round-robin mode, if no masters are requesting, the arbitration pointer will park on the highest
priority master if MPARK[PRK_LAST] is set, or will park on the master which last requested the bus if
cleared.
8.5.3 Bus Master Park Register (MPARK)
The MPARK controls the operation of the system bus arbitration module. The platform bus master
connections are defined as:
• Master 3 (M3): Fast Ethernet Controller (Not used for the MCF5216 and MCF5214)
• Master 2 (M2): 4-channel DMA
• Master 1 (M1): Internal Bus Master (not used in normal user operation)
• Master 0 (M0): V2 ColdFire Core
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3