JTAG (Flash)

JTAG – (Joint Test Access Group) is a tool used to read and write the firmware stored on the flash chip of many modern-day electronics. it’s actually a testing/troubleshooting interface built into the the motherboard at the factory. Hobbyists have been writing their own software so they can interface with these devices using their computers.

JTAG can also used to repair a router when it has been “Bricked”. This usually happens when you loose power in the middle of a firmware upgrade or flash an image that is incomplete. Using a JTAG, you can rewrite the flash on a device that is normally stuck in some sort of boot-loop or power-cycle. I’ve repaired several devices that would have otherwise been trash using a JTAG.

One of my favorite tech devices ever. Easy access to the flash via JTAG and the Boot-loader via Serial TTY provided endless hours of geeking out.
Typical JTAG header on an electronic device
JTAG, “the hard way”. Linksys did not include headers on the next generation WRT series router……resulting in lots of painful soldering to re-flash your device!  
Simple JTAG to re-write the flash on a WRT54GS router. This is simply on old printer cable with (10) 100 ohm resistors to provide protection from the computer port.
Simple JTAG connected to the header on the WRT54GS. The device allows you to read and write to the flash chip on the router. It’s useful if you want to put custom firmware like OpenWRT onto your router.
I built this JTAG device by looking at a store bought unit and reverse engineering it.
Rear view of my battery powered JTAG device that I built. This is one of the very first devices that I built in my home lab.
Here is another JTAG I built but this one uses power from the device you are working on. I got tired of replacing batteries or having them fail as they aged.
Rear view of my AC powered JTAG device.