JTAG Emulators/ Debuggers

[Ankur Kashyap]

hi

I'm working on ARM 7 based microcontroller. JTAG is now in every
chip...whether uCon,FPGA,DSP...ect.I would like to know what is JTAG
interface, how it works, pinout, how does emulators/ debugger with
JTAG work, while running the code...do the microcontroller sends
signals thru JTAG interface to debuggers...

Ankur

 

[Ken Smith]

hi

I'm working on ARM 7 based microcontroller. JTAG is now in every
chip...whether uCon,FPGA,DSP...ect.I would like to know what is JTAG
interface, how it works, pinout, how does emulators/ debugger with
JTAG work, while running the code...do the microcontroller sends
signals thru JTAG interface to debuggers...

There are a few logic lines:

TMS = Test mode select
TDI = Test data in
TDO = Test data out
TCK = Test clock

Inside the chip is a state machine and some shift registers.

Using TDI and TCK you shift bits into the various flip-flops inside. Some
of these flip-flops are grouped together into what is called a command
register. By setting the command register to different values, you cause
different things to happen.

The JTAG standard specifies some of these commands and some others are
defined by the maker of the chip.

Debuggers generally work with the commands the maker added. The chip
being debugged actually has most of the debugging circuits built into it.
The JTAG interface is just a way to get data to and from this hardware.