Wiggler (JTAG) - Widespread Uses

Widespread Uses

• Except for some of the very lowest end systems, essentially all embedded systems platforms have a JTAG port to support in-circuit debugging and firmware programming as well as for boundary scan testing:
  • ARM Architecture processors come with JTAG support, sometimes supporting a two-wire "SWD" variant or high speed tracing of traffic on instruction or data busses.
  • Modern 8-bit and 16-bit Microcontroller chips, such as Atmel AVR and TI MSP430 chips, support JTAG programming and debugging. However, the very smallest chips may not have enough pins to spare (and thus tend to rely on proprietary single-wire programming interfaces); if the pin count is over 32, there is probably a JTAG option.
  • Almost all FPGAs and CPLDs used today can be programmed via a JTAG port.
  • Many MIPS and PowerPC processors have JTAG support too
• The PCI bus connector standard contains optional JTAG signals on pins 1-5; PCI-Express contains JTAG signals on pins 5-9. A special JTAG card can be used to reflash a corrupt BIOS.
• Boundary scan testing and in-system (device) programming applications are sometimes programmed using the Serial Vector Format, a textual representation of JTAG operations using a simple syntax. Other programming formats include 'JAM' and STAPL plus more recently the IEEE Std. 1532 defined format 'ISC' (short for In-System Configuration). ISC format is used in conjunction with enhanced BSDL models for programmable logic devices (i.e. FPGAs and CPLDs) that include addition ISC_ instructions in addition to the basic bare minimum IEEE 1149.1 instructions. FPGA programming tools from Xilinx, Altera, Lattice, Cypress, Actel etc.. and will typically be able to export such files.
• As mentioned, many boards include JTAG connectors, or just pads, to support manufacturing operations, where boundary scan testing helps verify board quality (identifying bad solder joints, etc.) and to initialize flash memory or FPGAs.
• JTAG can also support field updates and troubleshooting.
• Primarily of historical interest: Intel's Pentium processors supported a "probe mode" supporting JTAG access for debuggers. For a long time, its documentation was withdrawn by Intel. Current x86 processors appear to use JTAG only for boundary scan.