Auto-increment Register, 0x18
This 32-bit register gives the increment value of the Comparator Register when the Auto-increment bit is set in the Timer Control Register. Each Cortex®‑A9 processor present has its own Auto-increment Register.
If the comp enable and auto-increment bits are set when the global counter reaches the Comparator Register value, the comparator is incremented by the auto-increment value, so that a new event can be set periodically.
The global timer is not affected and goes on incrementing.