Human Interface Device protocol

Human Interface Device protocol (HID protocol) is a USB protocol for a broad category of user input devices.

Devices in the category include but are not limited to keyboards, mice, pen tablets, webcams, headsets, game and simulation controllers.

HID protocol has a default polling rate of 125hz as compared to PS/2’s 100hz. This rate gives USB an edge in responsiveness with a lower latency. Both USB and PS/2 provide for faster speeds but where PS/2 tops out at 200hz, USB goes up to 1000hz, which works out to a tiny 1ms latency. This low latency provides excellent response time for keyboards, mice, VR headsets, gaming and simulation controllers.

With keyboards, HID protocol is used to both enable pre-operating system functionality with a 6 key rollover boot mode for BIOS (basic input/output system) and operating systems that are not USB aware. This mode has the caveat of interrupting the system every time the device is polled and it being polled regardless of whether there is a change in input or not. A separate operating system mode which enables further features does not have this issue. Many devices can function with basic drivers included with OSs until custom drivers are installed, making hardware installation easier.

HID protocol’s ability to announce its capabilities provides ease of connecting devices and having OS find the drivers makes USB a very plug and play experience. At the same time, the specification offers no means to verify that devices are what they claim to be. This lack of verification can be a vulnerability that leads to masquerading devices. BadUSB is an example of malware exploiting this vulnerability.