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.
See this entry on wikipedia Voting to close as its not confined to just Android and off-topic :)– t0mm13bJan 6, 2013 at 16:59
7I 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.– ce4Jan 6, 2013 at 17:22
2While 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.– FlowJan 6, 2013 at 17:51
1Wow 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.– hippietrailSep 12, 2015 at 13:35
1@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.– unforgettableidSupportsMonicaNov 11, 2015 at 4:26
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:
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.
JTAG is a Protocol !
Simply in other Protocols such as USB is protocol UART is protocol for transferring DATA or communicating with devices .
JTAG Directly communicate with the CPU and with the help of CPU's JTAG Protocol you can access CPU's peripherals and memory/NAND/eMMC one of this peripherals .
In order to relate directly answer to your question for the reviving part . Even Though your device is not able to boot-up from the internal memory/NAND/eMMC JTAG access still available through JTAG Protocol .
Accessing JTAG and re-writing Boot partitions to your device internal Memory/NAND/eMMC will revive your phone , phone will be able to boot-up and will be revived .
Some Commercial JTAGs : ort-jtag.com
Some JTAG Pinouts: