CODEWITHSUNDEEP
usbmicrocontrollerpackethid
alpemwarrior
alpemwarrior

Reputation: 53

How is it possible to send HID packets longer than 64 bytes?

While researching the HID specification for a current project, I stumbled across the following phrase:

Using USB terminology, a device may send or receive a transaction every USB frame (1 millisecond). A transaction may be made up of multiple packets (token, data, handshake) but is limited in size to 8 bytes for low-speed devices and 64 bytes for high-speed devices.

This leads me to believe that the maximum size for a HID packet is 64 bytes.

However, examining the report descriptors for some other devices I see that packets of more than 500 bytes are being used. How is this possible? Are these devices violating the USB spec? If so, what might that imply in terms of compatibility across different platforms?

Upvotes: 5

Views: 2347

Answers (1)

ocrdu
ocrdu

Reputation: 2183

As far as I know, the last HID specification is much older than the recent USB specs; IIRC the latest version is 1.11 which dates back to 2001.

There is low-speed, full-speed, and high-speed USB these days, and I think the HID spec was never changed to reflect this.

The maximum packet size for high-speed is 64 bytes for control transfers, 1024 bytes for interrupt transfers and isochronous transfers, and 512 bytes for bulk transfers.

See USB in a NutShell, which I think is more up-to-date. There are other sources, of course.

I am not entirely sure if all this applies to HID devices as well these days, what with the HID spec not having changed, but I assume high-speed HID devices exist now that use larger packets as described in the newer USB specs.

Upvotes: 5

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