Atmel AVR USB - Let's get cracking!

Recently purchased a Minimus USB clone as these have gotten relatively cheap thanks to all the current PS3 hacking (Disclaimer: I don't have a PS3 and don't care much about what other non-development use these boards are also being targeted at).

All this to say that getting an Atmel AT90USB162 based board, or similar, has now gotten cheap enough that you might as well get one, if you're planning to play with USB, using Atmel AVR chips.

The following post assumes that you're completely new to AVR development, and aims at helping you getting started. I'll assume the use of a Minimus USB board and Windows, for the rest of this guide.


Flip... flops?

Except for the bootloader, the flash should be pretty much blank on arrival, yet you probably found a test program that tests the LEDs or something, that you want to run to confirm that the board is good. There's an USB bootloader in there, to take care of the flashing, but to be able to use it you'll need the following:

1. the flashing application itself, which is Java based and called "Flip"
2. an USB driver for the application to use

If you head to the Atmel website you'll find the Flip 3.4.2 downloads. If you don't have Java installed already, you should obviously pick the one that has the JRE included, and install the application. That takes care of step 1, but we still need some drivers to have the app communicate with the device. These drivers are located in "C:\Program Files (x86)\Atmel\Flip 3.4.2\usb" (If you used the default installation path).
Now the good news here is that they are using the libusb-win32 Windows driver (smart choice!). The bad news however is that, as per Flip 3.4.2, they are not using the signed libusb0.sys drivers, or anything WHQL. Therefore, if you're running on Windows 7 or Vista x64, the driver installation will fail. Ouch!
Thankfully, you don't have to use the Atmel driver files from the Flip application, and the latest libusb-win32 ones, which are signed, will work just fine. The easiest way to install them then is to download the latest libusb-win32 binary release, extract the zip and then run the inf-wizard.exe you'll find in there, with your device plugged in. Or you can also download the latest Zadig.exe, select Options->Advanced Mode, and change the target driver from WinUSB. which is the default, to libusb0.

Once the driver is successfully installed, you can run Flip, and if you press Ctrl+U to bring the USB communications menu (or go to Settings -> Communication -> USB), you should now be able to open the device with Flip properly identifying your chip. Groovy.

I also should point out that a more detailed guide (with pictures) on how to use Flip with AT90USB chips is also available from Atmel.


Bootloader mode or not bootloader mode: that is the question

One item that might be of interest to you, if you're completely new to using bootloaders to reprogram the flash, is that once you successfully transfer an application to the flash and get it to run, the bootloader will obviously no longer be accessible, and if your app does not do anything with USB, it will be innaccessible from your computer.

So how do you get back to USB bootloader mode for reflashing? Simple:
Just make sure the HWB button is pressed when you press the RST button.

So in essence the method to enter the bootloader is 1. Press RST, 2. press HWB, 3. release RST, 4. release HWB - easy.
This of course assumes that the Hardware Boot Enable Fuse (HWBE) has been set on the board and that PD7 is connected to a HWB button. For more info, see chapter 23.5 of the AT90USB162 datasheet.


Is it Blinky? Or is it just Clyde?

"Enough of these preliminaries, let's get these LEDs blinking already!"

Alright, I hear ya. But if you assume that any LED blinking app listed on the Minimus USB website would work, you assumed wrong. The only one that actually successfully blinks the LEDs is the one you will find in the minimus_led_test.rar, and even that one, depending on how you run it, might not actually blink the LEDs because of how its program, but because of the watchdog timer (more about this in the next post).

So, provided you extracted the minimus_led_test.hex, and you have Flip in programming mode, the way to go is:

1. File -> Load Hex file and pick up the .hex. The FLASH Buffer information will now be populated
2. Click the 'Run' button, which will erase, program and verify the flash (default options). Note that if you try to flash without erasing first, you will get a "Device protection is set" error. You always need to erase the flash first
3. Click the "Start Application" button

Your LEDs blinking yet? Good. Now let's get ourselves sorted to write our own LED blinker app.
We'll setup a toolchain that generates Intel .hex files, and then flash these files using the same method above.