The target's JTAG interface is accessed using some JTAG-enabled application and some JTAG adapter hardware. There is a wide range of such hardware, optimized for purposes such as production testing, debugging high speed systems, low cost microcontroller development, and so on. In the same way, the software used to drive such hardware can be quite varied. Software developers mostly use JTAG for debugging and updating firmware.
If you want to acquire a JTAG adapter, you first need to decide what systems it must support. Everything else follows from that, including your software options. Low-end adapters may cost less than $US 50 and have limited hardware and software support. High-end adapters can cost a hundred times as much, including software support, and have corresponding improvements in capability.
ConnectorsEdit
A Netgear FVS336G firewall with a 14 pin JTAG header at lower left.
A Netgear DG632 ADSL modem with an 8 pin JTAG header at location "5".
There are no official standards for JTAG adapter physical connectors. Development boards usually include a header to support preferred development tools; in some cases they include multiple such headers, because they need to support multiple such tools. For example, a microcontroller, FPGA, and ARM application processor rarely shares tools, so a development board using all of those components might have three or more headers. Production boards may omit the headers; or when space is tight, just provide JTAG signal access using test points.
Some common pinouts[18] for 2.54 mm (0.100 in) pin headers are:
ARM 2×10 pin (or sometimes the older 2×7), used by almost all ARM based systems
MIPS EJTAG (2×7 pin) used for MIPS based systems
2×5 pin Altera ByteBlaster-compatible JTAG extended by many vendors
2×5 pin AVR extends Altera JTAG with SRST (and in some cases TRST and an event output)
2×7 pin Texas Instruments used with DSPs and ARM-based products such as OMAP
8 pin (single row) generic PLD JTAG compatible with many Lattice ispDOWNLOAD cables
MIPI10-/20-connectors (1.27 mm 050") for JTAG, cJTAG and SWD
Those connectors tend to include more than just the four standardized signals (TMS, TCK, TDI, TDO). Usually reset signals are provided, one or both of TRST (TAP reset) and SRST (system reset). The connector usually provides the board-under-test's logic supply voltage so that the JTAG adapters use the appropriate logic levels. The board voltage may also serve as a "board present" debugger input. Other event input or output signals may be provided, or general purpose I/O (GPIO) lines, to support more complex debugging architectures.
Higher end products frequently use dense connectors (frequently 38-pin MICTOR connectors) to support high-speed tracing in conjunction with JTAG operations. A recent trend is to have development boards integrate a USB interface to JTAG, where a second channel is used for a serial port. (Smaller boards can also be powered through USB. Since modern PCs tend to omit serial ports, such integrated debug links can significantly reduce clutter for developers.) Production boards often rely on bed-of-nails connections to test points for testing and programming.
Adapter hardwareEdit
Adapter hardware varies widely. When not integrated into a development board, it involves a short cable to attach to a JTAG connector on the target board; a connection to the debugging host, such as a USB, PCI, or Ethernet link; and enough electronics to adapt the two communications domains (and sometimes provide galvanic isolation). A separate power supply may be needed. There are both "dumb" adapters, where the host decides and performs all JTAG operations; and "smart" ones, where some of that work is performed inside the adapter, often driven by a microcontroller. The "smart" adapters eliminate link latencies for operation sequences that may involve polling for status changes between steps, and may accordingly offer faster throughput.
As of 2009, adapters with full speed USB links are probably the most common approach, and new products often include high speed USB support. Higher end products often support Ethernet, with the advantage that the debug host can be quite remote. Adapters which support high speed trace ports generally include several megabytes of trace buffer and provide high speed links (USB or Ethernet) to get that data to the host.
Personal computer parallel port adapters are simple and inexpensive, but they are relatively slow because they use the host CPU to change each bit ("bit banging"). They have declined in usefulness because newer computers do not have parallel port hardware. Driver support is also a problem, because the adapter electronics varied so widely.[citation needed]
Serial port adapters also exist, and are similarly declining in usefulness. They generally involve either slower bit banging than a parallel port, or a microcontroller translating some command protocol to JTAG operations. Such serial adapters are also not fast, but their command protocols could generally be reused on top of higher speed links.
With all JTAG adapters, software support is a basic concern. Many vendors do not publish the protocols used by their JTAG adapter hardware, limiting their customers to the tool chains supported by those vendors. This is a particular issue for "smart" adapters, some of which embed significant amounts of knowledge about how to interact with specific CPUs.
Software developmentEdit
Most development environments for embedded software include JTAG support. There are, broadly speaking, three sources of such software:
Chip Vendors may provide the tools, usually requiring a JTAG adapter they supply. Examples include FPGA vendors such as Xilinx and Altera, Atmel for its AVR8 and AVR32 product lines, and Texas Instruments for most of its DSP and micro products. Such tools tend to be highly featured, and may be the only real option for highly specialized chips like FPGAs and DSPs. Lower end software tools may be provided free of charge. The JTAG adapters themselves are not free, although sometimes they are bundled with development boards.
Tool Vendors may supply them, usually in conjunction with multiple chip vendors to provide cross-platform development support. ARM-based products have a particularly rich third party market, and a number of those vendors have expanded to non-ARM platforms like MIPS and PowerPC. Tool vendors sometimes build products around free software like GCC and GDB, with GUI support frequently using Eclipse. JTAG adapters are sometimes sold along with support bundles.
Open Source tools exist. As noted above, GCC and GDB form the core of a good toolchain, and there are GUI environments to support them.
All such software tends to include basic debugger support: stopping, halting, single stepping, breakpoints, data structure browsing, and so on. Commercial tools tend to provide tools like very accurate simulators and trace analysis, which are not currently available as open source.
