The Bus Blaster v1 is a prototype board (from Dangerous Prototypes) that allows to access the JTAG connection of integrated circuits through USB, using the FTDI FT2232H chip for the conversion. The prototype is on sale at around 35$ as they say on their page, but keep in mind that it is still a prototype, so I would advise buying it if you know how to solder and desolder, in case an error slipped in the PCB design and you need to fix it without re-buying the board. Still, the price is interesting since there are some commercial products in the range of 100$. Moreover the board design is open source, which is a nice addition and offers a starting point for hobbyists to begin developing their own debugging board or integrate the design into their own bigger board.
Connecting the Bus Blaster with the JTAG port of a supported microcontroller allows to debug the software running on it using GDB. This is possible also thanks to software like OpenOCD that translates debugging commands to JTAG sequences to send through the FT2232H to the microcontroller. It’s also possible to have more devices connected to the JTAG interface in daisy chain; this is common in board designs that mount more than one complex integrated circuits (for example a SoC and an FPGA).
- JTAG tutorial at fpga4fun
- List of debugging hardware supported by OpenOCD
- List of CPU cores supported by OpenOCD