Why USB Devices Must Enumerate First

USB is often treated as “plug in and communicate”. Keyboards, flash drives, USB serial adapters, cameras, and debuggers all use USB and appear to share one connector.

To understand USB, first notice how different it is from UART, I2C, and SPI: USB is usually not peer-to-peer. A Host manages Devices. After a device is plugged in, it cannot just start sending arbitrary data. It must first be discovered, identified, and configured by the Host.