JTAG (Joint Test Action Group) is a standard commonly used for debugging circuit boards and embedded systems.

In my last post I found out how to enter the bootloader, and execute some debugging commands through a serial console. However, it appears that the only way to write to the flash with the bootloader is with a proprietary protocol over raw ethernet with an unknown ethertype (0x8813). I have reverse-engineered it and written a flasher utility: sxflash (in their quest to make the bootloader as minimal as possible they decided not to simply go with TFTP over IP like most routers)...

I connected a FTDI USB to TTL 3.3v serial cable to the serial header on the device, and managed to get into the bootloader by a magic trick (holding the reset button at power-on and immediately releasing it). Using minicom, I got a kind of debug shell...

In this article I described how to use the signal generator firmware to send out PWM signals generated on the PC. However, there is a better way. The USBee SX is based on a Cypress FX2lp, a microcontroller for which the firmware must be sent every time the device is connected. This gives an excellent oppertunity to experiment with custom firmwares that do fun stuff, such as driving led effects independently from the host...

In the signal generator application packaged with USBee SX, every waveform has to be manually 'designed'. I wanted to implement a led fading "chase" effect using Pulse Width Modulation (PWM). It is far too much work to manually design the waveforms in that case. Also, as I wanted a solution that also works on Linux, I did not use the DLL API, but instead what I've learnt up to now about the USBee protocol and put it to use with Python and libusb, a convenient way to interface to USB devices without writing kernel modules or device drivers. This is the kind of effect I wanted to achieve (works browsers supporting HTML5 canvas only):

The signal generator program packaged with the USBee does not seem very intuitive to use on first sight. Initially I was unable to change anything, wondering why clicking anywhere in the waveform doesn't do anything. I was starting to doubt that the program was functional at all. Too bad, as this was my reason to get this device instead of the Salae.