Need to test your circuit-board for bugs or defects; or better yet, need to do in-system programming? Corelis JTAG testing tools allow you to do so; and if you’re into manufacturing electronics, their JTAG tools allow engineers to test, debug and program electronics, eliminating problems from the start, and reducing test developement costs and time to market.
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Update: Corelis now make a low cost bus analyzer with all the power, flexibility, and features you need to monitor and debug the I2C bus circuitry on your board. Find more about it here: Bus Analyzer
Also check out this informative blog on JTAG technology. http://boundaryscan.blogspot.com