After flashing a WGT634U router, which I wanted to convert to a 3g usb wifi router, a apparently used a wrong openwrt image, despite careful checking, the device was bricked. (I changed the header with 4 bytes, which I read …, which was not a good thing to do)
So a JTAG interface was needed to bring this device back to life. The first time in my life I would use this kind of interface.

Spending many hours on the internet what differences between JTAG interfaces are, how they are controlled, I found a very popular chip, the FT2232H from the company FTDI, which manifest itself into a myriad of circuits. This one is from Ebay: TIAO USB JTAG. Manual of this can be found here: http://www.tiaowiki.com

I am using an Apple mac since a long time, and considered to use it with that interface, but decided to do this on my Ubuntu based laptop (lubuntu 12.10). It would probably be easier. Drivers are already there, shell, network, what do I need more?
Once connected I checked via dmesg whether the device would be recognized:

marc@helium:~$ dmesg | grep usb
...
[13193.860094] usb 1-1: new high-speed USB device number 2 using ehci_hcd
[13193.997452] usb 1-1: New USB device found, idVendor=0403, idProduct=8a98
[13193.997461] usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[13193.997468] usb 1-1: Product: TIAO USB Multi-Protocol Adapter
[13193.997475] usb 1-1: Manufacturer: TIAO
[13193.997481] usb 1-1: SerialNumber: TIVYLJQO
[13194.212925] usbcore: registered new interface driver usbserial
[13194.213269] usbcore: registered new interface driver usbserial_generic
[13194.213633] usbserial: USB Serial Driver core
[13194.242320] usbcore: registered new interface driver ftdi_sio
[13194.245494] usb 1-1: Ignoring serial port reserved for JTAG
[13194.247381] usb 1-1: Detected FT2232H
[13194.247390] usb 1-1: Number of endpoints 2
[13194.247396] usb 1-1: Endpoint 1 MaxPacketSize 512
[13194.247403] usb 1-1: Endpoint 2 MaxPacketSize 512
[13194.247409] usb 1-1: Setting MaxPacketSize 512
[13194.249496] usb 1-1: FTDI USB Serial Device converter now attached to ttyUSB0