Advanced Peripheral Bus(APB) is a part of Advanced Microcontroller Bus Architecture(AMBA) specifications which define on-chip communications standard for designing high performance embedded microcontrollers.
The APB interface is designed for accessing the programmable control registers of peripheral devices. This specification will use Requester and Completer. APB bridges can also be referred to as a Requester. A peripheral interface responds to requests. APB peripherals can also be referred to as a Completer.
The state diagram shown is used to represent the activity of the peripheral bus.
This is a simple implementation to learn more about verilog concepts.