JTAG Connectors
There is no standard connector for JTAG. Typically, the “JTAG connector” is a standard male connector, such as 0.1″ header or a finer pitch connector. As we have seen, only four (or five) pins are required to operate the JTAG TAP. However, the device used to “communicate” with the TAP (called the JTAG interface) also requires a power and ground connection, and the designer can include other connections on the JTAG connector as needed.
So, given a board, how should a designer provide JTAG access? And, given a new board, where should you look for JTAG connectors?
Although there is no standard connector for the JTAG interface, several connector types have been more or less standardized by manufacturers. These include the ARM JTAG 20、ARM JTAG 14、TI JTAG 14、STMicroelectronics 的 STDC14、OCDS 16 Pin Header[pdf]、CoreSight 10、CoreSight 20、MIPI 34和Mictor 38 . Segger defines its J-Link and J-Trace connectors as nearly identical to the ARM JTAG 20.
Most fittings are male fittings with or without covers and are available in 10, 14, or 20 pin and 0.1″ or 0.05″ gauge. Examples are shown in Figure 1.
Figure 1. Universal connector for connecting the JTAG interface.
The pinout of the various JTAG interfaces (linked above) is shown in Figure 2. Here you will find the standard pins for JTAG (TDITDO, TCK, TMS, nTRST) as well as for serial line debugging (SWDIO, SWCLK, SWO)), as well as additional functionality for debugging, e.g. core tracing.
Figure 2. Pinout of various JTAG interfaces, in this case shown on a 0.1″ shrouded male header.
Of particular note among the added pins are nSRST (full system reset), which forces the target to be fully reset, and VTREF (voltage target reference), which connects to the target power rail of the JTAG interface hardware level translation.
JTAG Interface
There are several JTAG interfaces (also known as JTAG debug probes) on the market. In the open source hardware space, there is the Black Magic Probe or BMP, developed by 1BitSquared and Black Sphere Technologies, which is used as the ARM JTAG interface and has a large and active community supporting it. The Black Magic Probe can also refer to any JTAG interface that has had its firmware replaced with the Black Magic Probe firmware.
Segger’s widely used commercial debug probes include J-Link (shown in Figure 3) and J-Trace, a more advanced and powerful debug probe for industrial applications. J-Link sells for less than $100 under an educational license, $400 to $1,000 for commercial applications, and $1,700 to $2,500 for J-Trace.
Figure 3. Segger J-Link PRO Debug Probe and JTAG Interface
Specific vendors will also sell JTAG interfaces for their products; STMicroelectronics offers the STLINK family (including STLINK/N2 and STLINK-V3SET) for its STM8 and STM32 products, Atmel (now Microchip) offers the Atmel-ICE, NXP offers the S32 Debugging probes and the list goes on.
FPGAs also use JTAG to download bitstreams to devices/memory, but these interfaces are often called download cables. Examples include Xilinx’s Platform Cable II and Altera’s FPGA Download Cable, formerly known as USB-Blaster 1l, now renamed Intel FPGA Download Cable ll now renamed Intel FPGA Download Cable ll.
So what is happening with these devices that makes them so expensive? What features do they support and how do designers use them? Typically, if you look inside a low-end debug probe, you’ll find the following:
- A microcontroller as the primary JTAG controller
- USB interface, either embedded in the microcontroller or separately in e.g. an FTDI chip
- Level shifting circuits for logic compatibility
- Switching circuits for enabling and disabling different paths, pull-ups, etc.
That’s it. For example, check out the Black Magic Probe hardware documentation on Github. Most of the work (and cost) comes from the software side, providing powerful (and sometimes real-time) debugging tools that enable developers to take full advantage of the Arm CoreSight architecture.
Conclusion
At this point, we have covered the JTAG standard, including the Test Access Port (TAP) and its state machine. In this article, we examine the physical aspects of JTAG, looking at the connectors and interfaces available to designers from open source to commercial high-end.
From here, all that remains is to take a closer look at the Arm Core Sight architecture and its debug interface (ADI, which will include the increasingly common Serial Wire Debug (SWD) JTAG alternative.
Article from: allaboutcircuits
