AVR-USB-JTAG has all the power of the original AVR-JTAG plus the flexibility of a USB interface. This is a low-priced equivalent of the much more expensive JTAGICE, with a handy and safe optically isolated USB interface. Use it to debug and emulate many of the Atmel processors!
AVR In Circuit Emulator with USB-A Interface: [Link]
HappyJTAG2 is the new version of the HappyJTAG interface. Why a new version ? well it seems the old happyJTAG wasn’t powerful enough and there is a barrier in performance when manipulating bits trough USB interface. the USB traffic gets to huge to be effective. This is why HappyJTAG2 was created to bring more power over the same functionality. This version uses specialized FT2232 chip, which has integrated JTAG interface on chip.
This radically reduces USB interface traffic thus increasing the overall performance. There are two channels in FT2232, Channel A is used for JTAG and channel B is still free for standart serial communication between target and host PC. Unfortunatly, the FT2232 chip requires more components than FT232 to work. For example, external crystal oscillator, few more resistors and so on. But the result is balanced, because of overall performance is 100 times better if you compere with original HappyJTAG version.
HappyJTAG2 – JTAG AVR interface: [Via] – [Link]