Description
Avalon®-MM or AXI™ transactions that are not to a specified address in your Qsys system will be routed to the default slave. In version 13.0 and later of the Quartus® II software, you can choose the default slave.
When the default slave is not specified, the default slave will be one chosen out of the existing slaves that are connected to a master. The slave that is chose is undeteministic.
Resolution
To choose a slave component that will be the default slave in Qsys, follow the instructions below:
1) Right click on the title bar within the System Contents tab
2) Tick Show Default Slave column.
3) Any slave interface may be chosen as the default slave.