2.11.3. APB-Mux arbitration and APB Mux connectivity

This section describes APB-Mux arbitration and connectivity:

APB-Mux arbitration

The APB-Mux uses a fixed arbitration scheme to support the two slave interfaces. The arbitration logic ensures that only one APB bus master, either the APB-AP, or system bus master has access to the CoreSight Debug APB at any one time.

The APB-AP Slave port always has priority over the System Slave port. If two transfers are initiated at the same time, the transfer from the APB-AP Slave port is always propagated to the master port output for Debug APB access. The System Slave port is only granted access if the APB-AP Slave port is not requesting an access, that is, PSELAP is LOW. It is therefore possible for the APB-AP Slave port to maintain back-to-back transfers on the Debug APB without allowing access to the System Slave port. When a transaction is in progress on one slave port and the other port initiates a transaction, PREADY is held LOW for the newly requested transaction until the APB-Mux arbiter grants access to the other slave port.

The APB-Mux provides no address decoding, nor default response generation. This must be implemented by an external address decoder incorporating a default slave.

APB-Mux connectivity

The connection rules are:

  • AP-AP Slave port connects to the APB-AP

  • System Slave port connects to the system bus.

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