Arbitration Mode

The APB Interconnect core supports two arbitration mode:
  • Fixed-Priority—All enabled masters are granted the request based on the user-defined master priority in descending order. During the EVALUATION state, i.e., when apb_eval = 1, the APB Interconnect core evaluates all enabled masters' PSEL to decide which master is to be granted access. The master with Priority 1 gets the highest priority. Upon completing the requested APB transfer, if the same master with Priority 1 maintains its assertion of PSEL, it retains the priority. It is granted request every time it asserts PENABLE at one clock cycle after PSEL.
  • Round-Robin—Each master has the same rational order and takes turns to be granted requests in ascending order of their master numbering. For example, in the event of 3 masters, Master_0 is granted the request first, followed by Master_1 and Master_2. Then, the sequence restarts with Master_0.
Figure 1. Fixed-Priority Mode

In Fixed-Priority mode, transfer access is granted to the master with the highest priority and request (PSEL) signal.

Figure 2. Round-Robin Mode

For Round-Robin mode, see Figure 2. When all enabled masters are requesting, i.e., all s_apb_psel_i are asserted, every master is granted request one by one starting with Master_0, followed by Master_1, and Master_2.

Figure 3. Round-Robin with Skipped Master

If the master is not requesting upon its turn, the APB Interconnect core skips that master and grants access to the next requested master. However, the next requested master is served twice since it takes the previous master’s turn and serves its real turn.