What is JTAG?
JTAG is an IEEE standard (1149.1) developed in the 1980s to solve electronic boards manufacturing issues.
Nowadays it finds more and more use as IC debug or probing port.
Boundary testing
With the advent of the integrated circuits ("ICs"), boards are being populated with more and more ICs.
Here's a board with just two ICs, a CPU and an FPGA. A typical board may have many more ICs.
Nowadays ICs have more and more pins. Big ICs in BGA packages have over a thousand pins.
So of course, all these ICs are connected together with lots of connections (PCB traces).
We show just four here.
But you can easily have a few thousands.
Now if you build a thousand boards, each with a few a thousand connections, you inevitably have a few bad boards.
How do you test all these boards? You have to make sure that all these connections are fine.
You can't just test all these connections by hand. So JTAG was created.
JTAG can take control (or hijack) the pins of all the ICs. On the picture, maybe JTAG is going to make all the CPU pins outputs, and all the FPGA pins inputs. Then by sending some data from the CPU pins, and reading the values from the FPGA pins, JTAG can make sure that the board connections are fine. Doing that, JTAG test the PCB solders and the PCB traces for shorts and cuts, right from the ICs pins. So that a pretty good test. It is called an "IC boundary test".
Now JTAG really consists of four logic signal, named TDI, TDO, TMS and TCK.
From the PC's point of view, that's three outputs, and one input.
These four signals need to be wired in a particular way.
First TMS and TCK are wired in parallel to all JTAG ICs.
Then TDI and TDO and connected to form a chain.
In JTAG terminology, you often hear the term "JTAG-chain", that's where it comes from.
As you can see, each JTAG compliant IC has four pins used for JTAG (three inputs, and one output). A fifth pin named TRST is optional (JTAG reset).
The JTAG pins are usually dedicated (not shared for other purposes).
Nowadays, all big ICs can use boundary testing using JTAG. Boundary testing is the original reason JTAG was created.
Beyond boundary testing
But wouldn't it be nice to use JTAG for more purposes than just boundary testing?
- CPU and FPGA manufacturers allow JTAG to be used as debug port.
- FPGA manufacturers also allow configuring the FPGA through JTAG, and use the JTAG signals inside the FPGA core.
- Etc...
>>> NEXT: How JTAG works >>>
This page was last updated on July 25 2006.