How to write firmware for a custom USB keyboard?

Question

I have a custom ps2 keyboard(8x8 matrix) interfaced with AT89C51ED2 microcontroller, now I need to change it to USB interface. I have been studying about the basics of USB HID class communication(USB HID class specs, USB complete, Beyond logic) and have come to know little bit about the theory behind it.

But I am not able to understand the firmware part, I read a demo keypad application by Microchip which had given a sample source code, but I am not able to understand in the code how data(key pressed) is sent to the IN endpoint and how the host reads that through polling. I know that endpoint is a buffer from which data is sent to host, but Is it one of the registers of the micro-controller and how do I use it in my code?.

And I have been searching the suitable micro-controller with USB support, but no supported demo's are available, any suggestions will be helpful

Answer 1

The AT89C51ED2 datasheet does not mention hardware support for USB, so the answer is no: an endpoint does not correspond to a hardware register. Instead, an endpoint would refer to some software buffers in the RAM of the chip, and some data to keep track of the endpoint's state. Every bit of every USB packet must be handled by the firmware of the device, and endpoints are an abstraction that live entirely in the firmware of your device.

Note: I am assuming that the keyboard doesn't have some kind of USB interface chip, and that the data lines of the USB cable connect more-or-less directly to the microcontroller.

If you can't find the source code for the keyboard, you might look into using an open-source AVR software USB implementation. Here are some useful links:

• https://www.obdev.at/products/vusb/
• http://www.fischl.de/usbasp/

Please note that you are undertaking an advanced project, and if you are not familiar with AVRs, USB, microcontrollers, reverse engineering, and embedded development/debugging, it might be useful to start with something simpler first.