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USB-HID’s main capabilities

USB-HID’s main capabilities
The exchanged data is stored in a structure called a report report, and the device’s firmware must support the format of the HID report. The host transmits and requests reports in control and interrupt transfers to transmit and receive data. The format of the report is very flexible and can handle any type of data.
Each transaction can carry small or medium amounts of data. The low-speed device has a maximum of 8 bytes per transaction, the full-speed device has a maximum of 64 bytes per transaction, and the high-speed device has a maximum of 1024 bytes per transaction. A single report can use multiple transactions.
The device can transmit information to the host at unexpected times, such as keyboard keys or mouse movements. So the host will periodically poll the device to get the latest data.
Main limitation
The maximum transmission speed is limited, especially at low speeds and full speeds. The host can guarantee low-speed interrupt endpoints, there will be no more than one transaction per 10ms, and up to 800 bytes per second. The host can guarantee a full-speed endpoint with 1 transaction per 1ms and a maximum of 64,000 bytes per second. The host can guarantee high-speed endpoints, with a maximum of 24.576MB per second for every 125us1 transaction.
There is no guaranteed transfer rate. If the device is set to a time interval of 10ms, the time between transactions may be equal to or less than 10ms. Unless the device is set to transmit data at each frame at full speed, or at every high frame at high speed. This is the fastest polling rate, so the breakpoints guarantee that the correct bandwidth is available.
The Windows 98 Gold system does not support interrupt output transmission. All host and device data must use control transfer.
In addition to transmitting data to the host, the HID device also receives data from the host. As long as the device that can meet the HID category specification can be an HID device, examples such as a bar code pen, a thermometer, and a voltmeter are mentioned in the specification. Although these devices do not have a traditional human-machine interface, they can all transmit data to the host or receive configuration requests from the host.
In addition to the HID interface, the device may also contain other USB interfaces. For example, the image display device may use the HID interface to perform brightness, contrast, and update rate software control, and use a conventional image interface to transmit data to be displayed. USB speakers can use real-time transmission to play voice, while using the HID interface to control volume, oscillating, and bass. HID interfaces are usually cheaper than traditional control interfaces.
The specification for HID category devices is Device Class Definition for Human Interface Devices. There is also a file, HID Usabe Tables, which defines the values ??that let the host know and use the HID data. The two documents were developed by the USB Device Working Group, and members of the Group are members of the USB Implementers Forum. You can download these two files on the USB Implementers Forum website.

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