Simple analysis of the USB driver
To start with, after reading this article, you may not fully understand the USB driver. However, the purpose of this text is to provide some key insights and ideas that might help you in your learning journey.
I. Overall Framework
II. Hardware Foundation
The hardware of a USB connection is relatively simple. It consists of four wires: one for power, and two data lines (D+ and D-). Though it looks straightforward, these components play a crucial role in establishing 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-. When a device is plugged in, the device itself uses a 1.5KΩ pull-up resistor on either D+ or D-. This causes the corresponding line on the PC side to go high, signaling the presence of a new device. This process is fundamental to how the system detects and initializes USB peripherals.
III. Software Concepts
1. Overview of a USB Device
From the image, we can see that:
- Devices usually have one or more configurations.
- Each configuration may contain one or more interfaces.
- Interfaces can have multiple settings.
- Each interface may have zero or more endpoints.
2. Endpoints
Endpoints are the basic building blocks of USB communication. They only support unidirectional data transfer. The direction is relative to the host: an output endpoint sends data from the host to the device, while an input endpoint sends data from the device to the host.
There are four main types of endpoints:
Control Endpoint: This is used for managing access to different parts of the device. It’s commonly used for device initialization, retrieving information, sending commands, and receiving status reports. Data transfers here are reliable and guaranteed in timing.
Interrupt Endpoint: These endpoints are used for small, periodic data transfers at fixed intervals. Unlike hardware interrupts, they are initiated by the host, not the device. They are typically used in devices like mice and keyboards.
Bulk Endpoint: These are used for transferring large amounts of data where timing is not critical. Examples include USB drives and other storage devices.
Isochronous Endpoint: These also handle large data transfers but do not guarantee delivery. They are often used in real-time applications such as microphones and video streaming.
3. Interface
An interface represents a single logical function within a USB device. For example, a USB webcam might have a microphone built-in. To support both functions, the device would require two separate interfaces, each handled by its own driver. This shows that one physical device can have multiple drivers, depending on the functionality it provides.
4. Configuration
A USB device can have multiple configurations, allowing it to switch between different modes. For instance, a device that supports firmware updates may use different configurations to enable or disable certain features. Only one configuration can be active at any given time, ensuring the device operates correctly without conflicts.
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