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I see the term JTAG used for reviving dead Android devices that otherwise don't respond to anything and aren't seen when USB connected. It seems that it's something done from the physical hardware side of the device (as opposed to some pure software based solution) and I've heard the term used elsewhere (in gaming consoles). I don't find much else about it, and I'd like to know specifically how it relates to Androids and similar devices. It seems to be a term in electronics or electrical engineering, both of which I am unfamiliar with. I'm wondering what it is and why it can be used on hard bricked devices.

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See this entry on wikipedia Voting to close as its not confined to just Android and off-topic :) – t0mm13b Jan 6 '13 at 16:59
I find this to be an on-topic and legitimate question. @t0mm13b: You're right, this is some layers below Android on the hardware level (and JTAG is used by many embedded hardware products from broadband/wifi routers to microcontrollers for washing machines, etc.). However because this abbreviation pops up more frequently in relation to Android (e.g. on XDA unbrick howto's), one may ask if there's a connection between those two. – ce4 Jan 6 '13 at 17:22
While JTAG is not an Android specific thing, I still like the idea of having a question and answer(s) that explains how JTAG is used wrt to Android. – Flow Jan 6 '13 at 17:51
Wow this is the first mature SE site I've interacted with that doesn't have a terminology tag! I highly recommend creating one for questions such as this. – hippietrail Sep 12 at 13:35
@hippietrail: Thanks for the suggestion. I've now created the tag. Please feel free to suggest edits which will add it to other Android.SE questions. – unforgettableid Nov 11 at 4:26

1 Answer 1

JTAG and Android are two separate things.

You may find JTAG pins visible either under your device's battery or near its SIM / MicroSD card holder. The pins are hidden. You may have to dismantle your phone's back cover. The JTAG pins are usually six gold pins clustered close to each other, as shown in this photo:

Back of Zte Blade
Click to enlarge

You use a special hardware JTAG box which comes with a vast array of cables (different cables for different types of devices), plus specialized software specifically made for your device model. (There exist generic catch-all JTAG boxes that can be loaded with specific microcode instructions for your particular manufactured board. The tricky part is getting the exact microcode for your device's circuit board. If you use the wrong microcode, it can cause permanent damage.)

The microcode instructions are emitted by the JTAG box attached to the cable, clipped in position on top of the JTAG pins. The box sends a signal to your device to revive it. Now an appropriate firmware image can be flashed to your device.

This is a highly specialized topic. A background in electrical engineering can be handy, to know which pins are used for ground (GND), transmit (TX), receive (RX) and power (PWR). The power is the important one: if the battery is dead, power can still be fed into your device.

You can buy JTAG boxes online, but they tend to cost a couple hundred US dollars or so. It's because of differing manufacturers with different types of cables, and different arrangements of on-device JTAG pins.

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