Simple analysis of the USB driver
Before diving into the details, I'd like to mention that after reading this article, you might not fully grasp the entire concept of USB drivers. However, the purpose here is to provide some key ideas and insights that may help you better understand how USB drivers work.
I. Overall Framework
II. Hardware Foundation
The hardware of a USB interface is quite simple, consisting of four wires: one for power, and two for data (D+ and D-). Although it seems straightforward, these components play a crucial role in communication between devices.
Identification Process: As shown in the diagram, when a USB device is not connected, the PC’s USB port has pull-down resistors (15KΩ) on both D+ and D-. On the device side, either D+ or D- is pulled up with a 1.5KΩ resistor. When the device connects, the voltage is shared through the two resistors, causing the D+ or D- line on the PC side to go high. This change signals the PC that a new device has been connected.
III. Software Concepts
1. Overview of USB Devices
As illustrated, a USB device typically contains one or more configurations. Each configuration can have multiple interfaces, and each interface can have multiple settings. Interfaces, in turn, are associated with endpoints — which are the actual data transfer points.
2. Endpoints
Endpoints are the fundamental building blocks of USB communication. They support data transfer in only one direction. From the host's perspective, an endpoint can be either an output (host to device) or an input (device to host).
There are four main types of endpoints:
- Control Endpoint: Used for managing access to different parts of the device. It is used for device initialization, retrieving information, sending commands, and receiving status reports. These transfers are reliable and time-guaranteed.
- Interrupt Endpoint: Transmits small amounts of data at fixed intervals. Unlike hardware interrupts, these are initiated by the host. They are commonly used in devices like mice and keyboards.
- Bulk Endpoint: Designed for transferring large amounts of data, but without guaranteed timing. These are often used in storage devices like USB drives.
- Isochronous Endpoint: Also used for large data transfers, but they do not guarantee delivery. These are ideal for real-time applications such as audio and video devices.
3. Interface
An interface represents a single logical connection within a USB device. For example, a webcam might also include a microphone. To support both functions, the device needs two separate interfaces — each corresponding to a specific function. This shows that a single physical device can have multiple drivers, depending on its functionality.
4. Configuration
A USB device may have multiple configurations, allowing it to switch between different modes. For instance, some devices allow firmware updates and use different configurations to manage this process. Only one configuration can be active at a time, ensuring stable operation.
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