APB Master
The APB agent that initiates transactions — drives address, data, and control signals, and waits for the slave's ready.
Detailed Explanation
On the APB bus there is exactly one active initiator at any given time, and that role is the APB master. The master holds the address, data, and write-enable lines and walks every transaction through the two APB phases: a *setup* phase where `psel=1, penable=0`, and an *access* phase where `penable=1` and the slave must respond before the master can deassert. Only after the slave asserts `pready` does the transaction complete.
Masters tend to be simpler than their AXI counterparts because APB has no outstanding transactions, no bursts, and no reordering — every cycle either drives a transfer or waits for one to finish. That simplicity is the reason APB dominates configuration-register access in SoCs: the master is small, the protocol is easy to verify, and the bus is low-frequency relative to the main fabric.
Industry Context
In practice an APB master is usually an AXI-to-APB bridge sitting between the SoC's main fabric and a peripheral cluster, rather than a standalone component.