When an iPhone connects to a Mac mini for a DFU restore, the first conversation isn't about data. It's about power. This silent negotiation happens almost instantly over two specific pins in the USB-C connector: the Configuration Channel, or CC lines for short. These lines are the backbone for establishing roles and power levels before any firmware flashing can begin.

The Mac mini, acting as the power source, uses a pull-up resistor ($R_p$) to advertise a specific current capability on the CC line. The iPhone, the power sink, uses a pull-down resistor ($R_d$). The resulting voltage on the CC line, determined by this simple voltage divider, signals the iPhone's presence. This initial electrical handshake allows the Mac to detect that a device is connected and what its basic power requirements are, all before any complex digital communication starts.

Once presence is detected, the negotiation gets more sophisticated. This is where USB Power Delivery (USB-PD) comes in. It's a protocol that runs over the CC line, allowing for a back-and-forth conversation. The Mac mini broadcasts a list of its available power profiles—different combinations of voltage and current it can supply.

The iPhone, even in DFU mode, must respond. Its Power Management Integrated Circuit (PMIC) is awake and handles this task. The PMIC is a specialized chip that manages the phone's power rails, battery charging, and boot sequences. It analyzes the Mac's offered profiles and sends back a request for the one it needs to proceed with the DFU process. This negotiation ensures the iPhone receives stable and sufficient power before attempting to initialize more power-hungry components.

The CC line also determines which device supplies VCONN, a separate low-voltage power source. VCONN is used to power electronics inside the USB-C cable itself, which can contain small chips for identifying cable capabilities or managing signal integrity. For the DFU connection, ensuring both VBUS (the main power line) and VCONN are stable is critical for maintaining a reliable link.

The iPhone's power needs in DFU mode differ significantly depending on its prior state. This leads to two distinct power profiles.

A cold boot occurs when the device starts from a completely powered-off state. The initial power draw is minimal, as the PMIC only needs to power itself and a few essential circuits to listen on the USB port. The power negotiation is clean and starts from the lowest default USB power state before ramping up.

A warm boot, or a forced restart into DFU mode from a running OS, is more complex. The PMIC must first safely power down other components before re-initiating the DFU boot sequence. This can cause slight fluctuations in power draw on the VBUS line as different subsystems are shut down. The subsequent power negotiation might happen more quickly, as some state information could be retained by the PMIC.

Understanding this initial power-up sequence is fundamental. It ensures the iPhone's hardware is in a stable, receptive state, with the PMIC ready for commands, even while the main application processor remains dormant. This electrical foundation makes the subsequent data transfer and firmware flashing possible.