A hands-on guide to turning an nRF52840 development kit into an iPhone hands-free controller using the Zephyr RTOS. Covers two BLE protocols: HID (Human Interface Device) for emulating a BLE keyboard to send media keys (volume, play/pause, next/previous track) and simulate a 2-second power button hold to launch Siri, and Apple Notification Center Service (ANCS) for receiving incoming call notifications and sending accept/decline actions. Includes HID report map byte definitions, bt_hids_inp_rep_send usage, ANCS client initialization, notification subscription, and callback implementation with code snippets throughout.
