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How to use JTAG? Any tutorials?

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Hi,

I'm trying to learn how to use the JTAG to do various things that JTAG is capable of in terms of hardware hacking.(Read out/Write Firmware, Debug with JTAG, monitor data flow, etc.) I've had no previous experience with JTAG but I'm determined to learn all the knowledge required and buy any tools necessary. For the record, my I graduated with a degree in EECE, but most of the things I did in school was very theoretical stuff and I don't have much practical experience. Also, I haven't really had any experience in embedded systems.

Can someone point me to a link or document where I could understand how to actually use(not just theory, but also the practical usage and stuff) the JTAG, and what pieces of hardware and software do I need? Would be great if there was something in a tutorial form, but if none exists, any kind of material would greatly help.

Also, it would be great if you could recommend a cheap electronic device that has JTAG pins on it so I could possibly do some hands on experience.

Thanks.
 

JTAG is utilized by vendor specific tools, e.g. for programmable logic devices and processors. You'll find the respective information with each vendors documentation.

There are also general purpose JTAG hardware and software tools for development and test purposes. See some (mostly commercial) examples:

TopJTAG Probe (formerly Scanseer) — Boundary-Scan (JTAG) Based Circuit Debugging Software
JTAG Live free debug tool - Home
Introduction to JTAG / Boundary Scan - Resources and Applications
**broken link removed**
**broken link removed**
JTAGTest: Affordable JTAG testing and development solution
 
Thanks for the reply. :)

So I would have to read the manual of the device that I'm looking into in order to know what exactly I have to do in order to use the JTAG interface right?
I was thinking the JTAG interface would usually be standard across all devices but I guess they're not seeing your reply.

Also, if I can find no JTAG pins on the circuit board, does that means I have lost all hope in being able to use JTAG and I would have to find other means to debug the device?

Thanks.
 

So I would have to read the manual of the device that I'm looking into in order to know what exactly I have to do in order to use the JTAG interface right?
There are vendor specific and general JTAG functions. From the latter, boundary scan functions can be expected for most JTAG enabled devices, but you have to refer to the vendor documentation, particularly the BSDL (boundary scan descrition language) file, a machine-readable representation of this information. The mentioned general purpose JTAG tools are using this data when displaying pin logic levels or stimulating outputs.

Of course, the JTAG interface has to be exposed at a connector to use it.
 

Thanks.

So does that mean I would know what pins represent what JTAG pin by looking at the device manual? And after that, I would connect the JTAG debugging device pins to the respective pins on the device that's being analyzed right? And then follow the descriptions on the JTAG debugging device manual an so on...

Also, people say that many end products just leave the JTAG points on the PCB, but to me, all the vias and pins look the same and I don't really know how I would go about differentiating a JTAG via/pin from an original pin. Do you have any advice on how to find out which pins or vias actually represent the JTAG ports?
 

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