I have written USB device firmware for AVR, and read docs for a bunch of microcontroller families' with different internal HID device units that I wanted to port to.
If you do take suggestions for your next version, there are some hardware features that I would like:
* Let the MCU replace the next unsent input report, and atomically. Many USB devices can only queue reports, not replace them.
* Allow the microcontroller to know when a input report has been received. (ACK)
The first would make it possible to get the lowest latency with reports such as keyboard reports that contains only the state of momentary switches.
The second would make it possible to ensure that an event has been received, even if the host would poll at a slower rate than what the firmware works at.
The situation is especially complex for mice, where the reports have inputs that are relative — to the previous report.
* Let the MCU replace the next unsent input report, and atomically. Many USB devices can only queue reports, not replace them.
* Allow the microcontroller to know when a input report has been received. (ACK)
The first would make it possible to get the lowest latency with reports such as keyboard reports that contains only the state of momentary switches. The second would make it possible to ensure that an event has been received, even if the host would poll at a slower rate than what the firmware works at.
The situation is especially complex for mice, where the reports have inputs that are relative — to the previous report.