2.2.2. Least Recently Granted (LRG) scheme

In the LRG scheme, each connected SI has a single slot associated with it, but each interface also has a priority value. This priority value, whose post-reset value can be configured at design time and programmed or interrogated through the APB programming interface, can make the arbiter behave as:

All masters with the same priority form a priority group. As a result of arbitration, a master can move within its priority group but cannot leave its group, and no new masters can join the group.

Arbitration is granted to the highest priority group from which a member is trying to win access, and within that group, to the highest master at that time. When a master wins arbitration, it is relegated to the bottom of its group to ensure that is cannot prevent other masters in its group from accessing the slave.

If you configure all master priorities to different levels, the arbiter implements a fixed priority scheme. This occurs because in this case, each master is in a group of its own, and therefore, masters maintain their ordering.

If all master priorities are the same, then an LRG scheme is implemented. The reason that it behaves as an LRG is because the process of relegating the master that was last granted access, to the bottom of its group, results in the masters being ordered from the LRG master at the top, to the Most Recently Granted (MRG) at the bottom.

The LRG and fixed priority modes concurrently exist when the master priority value registers are programmed with a combination of identical and unique values. You can mix priority groups that contain one member with priority groups that contain more than one member in an arbitrary manner. The arbiter places no restriction on the number of groups or their membership.

Figure 2.3 shows the movement of masters within their priority groups.

Figure 2.3. Example operation of LRG arbitration scheme

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