Intel 80312 I/O Companion Chip - ECEE

Intel ® 80312 I/O Companion ChipIntel ® 80312 I/O Companion Chip ArbitrationTable 7-1.Bus Master / Programmed PrioritiesBus MasterProgrammed PriorityIntel ® 80312 I/O companion chip Bridge High - 00 2Device 0 Medium - 01 2Device 3 Medium - 01 2Device 4 Medium - 01 2Device 1 Low - 10 2Device 2 Low - 10 2Device 5 Disabled - 11 2Table 7-8 shows the 2-bit values that correspond to each priority level. A priority level of 11 2effectively disables the associated device by removing it from the arbitration sequence. A deviceprogrammedwitha11 2 priority never receives a grant to gain access to the bus.The priority of the individual bus master determines the level to which the device is placed in theround-robin scheme. The programmed priority determines the starting priority or the lowest prioritythe device is. If the application programs the device for low priority, the device may be promoted upto medium and then high priority until it is granted the local bus. Once the SARB grants the bus andthe device asserts S_FRAME#, the device is reset to its initially programmed priority.Note:If a low priority master requests the bus and there is no other higher priority agent requesting the bus, thatmaster is granted the bus the following clock. The promotion mechanism does not consume bus cycles.The round-robin arbitration scheme supports three levels of round-robin arbitration: low, medium,and high priority. Using a round-robin mechanism ensures there is a winner for each priority level.To enforce the concept of fairness, a slot is reserved for the winner of each priority level (except thehighest) in the next higher priority level. When the winner of a priority level is not granted the busduring that particular arbitration sequence, it is promoted to the next higher level of priority.7.2.1.2 Priority Example with Three Bus MastersTable 7-2 presents an example of bus arbitration with three bus masters:Table 7-2.Bus Arbitration Example – Three Bus MastersPriorityLevelInitialStateEach of the bus masters (A, B, and C) are constantly requesting the bus and each is at a differentpriority level. The top row of Table 7-2 lists the current bus master/winner of the highest prioritygroup. The three rows labelled as high, medium and low represent the actual priority levels thatdevices are currently at based on either their initial programmed priority or promotion through thelevels. For example, device C starts out at low priority. Because it is the only device at this priority, itis the winner at low priority and is promoted to medium priority. Later, it wins at the medium prioritylevel (against device B) and is promoted to high priority where it wins the level (against device A)and the bus. Device C is then put back at its programmed priority of low and starts the cycle over.Continuing with Table 7-2, the winning bus master pattern follows as:ABACABACABACABACWinning Bus MasterA B A C A B A CHigh A B A C A B A C AMedium B C C B B – C B BLow C – – – – C – – –NOTE: In this example, all bus masters are continually requesting the bus.7-4 Developer’s Manual