Why do writes to IP without a write interface get routed to the Default Slave in a Qsys System?

Description

Certain IP in Qsys systems can be instantiated without a write interface, for example, input-only PIO or System ID peripherals.  Writes issued to the addresses mapped to such IPs will be issued to the default slave, and if no default slave is explicitly specified in a Qsys system, Qsys will choose the IP with the largest address space.

Resolution

Always specify an explicit default slave where arbitrary writes cannot harm system operation, such as a small on-chip memory.