Ever wonder what those JTAG ports on your microcontroller are good for besides just a way to download software? Basically JTAG gives you the equivalent of the physical test points that used to be present on printed circuit boards before the days of multi-layer boards. ChipCenter.com has published a very good JTAG tutorial. And, if you want more detail, TI offers a 145 page JTAG Primer (PDF format).