JTAG

The Joint Test Action Group (JTAG) standardized a 5 pin boundary-scan test port as IEEE Std. 1149.1, "Standard Test Access Port and Boundary-Scan Architecture".

While originally intended for boundary-scan testing of PCB assemblies, to replace bed-of-nails testing, the JTAG port included in many popular microprocessors is also often used for programming and debugging.

"If you have information on how to connect a JTAG probe to a Nokia phone, please let me know."

There are five pins:

• TCK/clock
• TMS/mode select
• TDI/data in
• TDO/data out
• TRST/reset (optional), when driven low, resets the internal state machine.

Except for TCK, all other JTAG lines should be pulled high via a resistor.

WARNING: unconfirmed pinout. Please add links to pinout standard.

20 Pin JTAG PinOut

Which one of these is right?

   Pin Function Pin Function
   1   TRST     2   GND
   3   TDO      4   GND
   5   TDI      6   GND
   7   TMS      8   GND
   9   TCK     10   GND
   11  VPP_E   12   GND
   13  A/W     14   GND
   15  User 0  16   GND
   17  Rdy/Bsy 18   GND
   19  User 1  20   Vcc

    1 +3.3 V    2 +3.3 V
    3 nTRST     4 GND
    5 TDI       6 GND
    7 TMS       8 GND
    9 TCK      10 GND
   11  --      12 GND
   13 TDO      14 GND
   15 nRST     16 GND
   17  --      18 GND
   19  --      20 GND 

external links

• the OpenJTAG wiki ( http://openjtag.net/ )
• "When designing development boards what style JTAG connector should I use?" The 20-pin JTAG connector.
• "Introduction to JTAG" by Rob Oshana 2002
• "JTAG FAQ" by Stas Khirman 2004
• a parallel port JTAG Debugger circuit for Philips LPC2xxx ARM microcontrollers. by K9JTAG (uses Schmitt trigger inverters, so it can connect 3.0 V target boards to a standard 5.0 V parallel port)
• parallel port JTAG "Building Simple JTAG Cable" (resistors only)
• "JTAG Bus Description"
• Wikipedia:JTAG
• OpenWRT wiki: JTAG Cables
• the Jtag-Arm9 project at Sourceforge gives instructions and photographs of a Home made JTAG interface (also shows an example of prototyping using SMT IC)
• the JTAG protocol by Mark Whitis
• "Building AVR Jtag clone" includes schematics and firmware.
• Embecosm publishes a "SystemC JTAG interface specification" to simplify debugging complex chips.

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