When debugging Cortex-based microcontrollers, having the right debug connector is crucial for efficient and effective debugging. The debug connector provides the physical interface between the target Cortex device and the debugger/programmer. With a variety of debug connectors available for Cortex devices, choosing the right one can be confusing. This article provides tips on how to select the optimal debug connector for your specific Cortex application.
Understand Debug Connector Basics
Debug connectors for Cortex devices typically use the Joint Test Action Group (JTAG) or Serial Wire Debug (SWD) protocols to communicate with the target device. JTAG uses a dedicated test access port (TAP) controller on the chip, while SWD shares the processor’s system bus. JTAG requires more pins than SWD, but provides additional capabilities like boundary scan testing.
For most debugging purposes, SWD is sufficient and uses only two pins – Clock (SWCLK) and Data (SWDIO). Some processors like Cortex-M0/M0+ only support SWD. Older Cortex-A processors support JTAG, while newer Cortex-A and all Cortex-R/M processors support both SWD and JTAG.
The 10-pin Cortex debug connector defined by ARM provides all the necessary JTAG/SWD signals for debugging. Vendors like Tag-Connect and Picroft provide off-the-shelf cables with different styles of 10-pin connectors. However, for custom boards, smaller connectors can be used based on debug needs.
Determine Debug Interface Type
The first factor in choosing a debug connector is determining which debug interface your Cortex chip uses – JTAG or SWD. While most Cortex chips support SWD, some legacy processors may be limited to JTAG. Refer to the processor’s datasheet or ARM documentation to verify which debug modes it supports.
For example, Cortex-M3/M4/M7 microcontrollers mostly use the SWD interface. On the other hand, older Cortex-A8 applications processors support JTAG-only debugging. Knowing the type of debug interface narrows down the connector options.
Evaluate Target Board Space and Routing Constraints
Connector choice also depends on the available space on the target board and PCB routing constraints. Larger 10-pin connectors may not fit well on boards with tight spacing. In that case, smaller connectors like Tag-Connect’s 6-pin interface can be used with only SWD signals.
Carefully measure vacant space on the board to determine the maximum connector footprint size possible. Also assess whether the board provides a straight routing path from the connector footprint to the target Cortex IC. Opt for right-angle connectors if board layout makes routing difficult.
Match Connector Pitch to Board Stackup
Connector pitch, or distance between connector pins, must match the circuit board stackup – specifically the distance between adjacent holes. Most Cortex dev boards have 1.27mm (0.050″) pitch connectors. Ensure the connector you choose has the same pitch as board holes to allow proper alignment.
For example, Samtec’s FTSH series provides vertical board-stack debug connectors with 1.27mm pitch in 6, 10, and 20-pin options. Similarly, Molex Preci-Dip clip connectors come in fine-pitch 0.8mm to 2.54mm versions.
Choose Connector Plating Based on Durability Needs
Connectors used for Cortex debugging often need to withstand multiple mate/unmate cycles during development. Gold plated connector pins provide maximum durability for this usage. Nickel plating offers moderate durability, while tin-plated connectors are lowest cost but wear out quicker.
For frequently used debug connections, choose connectors with 30u” hard gold plating like Tag-Connect TC2050 or Samtec FFSD series. For lightly used connections, 15u” min gold or nickel plating like Harwin EZ-Board connectors can be cost-effective.
Select Number of Pins Based on Debug Mode
If the target Cortex processor uses SWD-only, then only 2 pins are needed – SWCLK and SWDIO. However, additional pins can provide breakout access for ETM trace, device reset, voltage sensing, and more. Here are typical pin configurations:
- 2-pin – SWCLK, SWDIO (SWD only debugging)
- 4-pin – SWCLK, SWDIO, GND, VCC (power and ground)
- 6-pin – SWCLK, SWDIO, GND, VCC, TRST, TDI (JTAG support)
- 10-pin – Full ARM 10-pin with SWD/JTAG signals
- 20-pin – Adds breakouts for ETM, reserves, I/Os, etc
Choose the minimal connector pin count to support required debug features. More pins add complexity but enable additional debug capabilities.
Get Tool Provider Recommendations
Silicon vendors like NXP, STMicro, and Texas Instruments often provide debug connector recommendations based on the evaluation tools they offer. For example, NXP’s LPCXpresso boards use 10-pin JTAG/SWD connectors.
Consult your Cortex tool provider’s guidelines on optimal connectors to match their debug probes. This ensures the connector you choose is fully compatible with their debugger hardware and software.
Check Connector Pricing and Availability
Compare pricing across shortlisted connector options to determine the most budget-friendly choice. Be aware that advanced connectivity technologies like coaxial or optical-based debug connectors can be significantly more expensive.
Also, check connector availability and lead times, especially for newer technologies. Opt for connectors with stable supply and shorter lead times to avoid delays or shortages during product development.
Evaluate Ease of Use and Other Features
Consider how frequently the debug connector will need to be used and by whom. Opt for easier to use connectors with locking tabs, color coding, and clear polarity markings if non-experts will frequently connect them.
Also, evaluate additional connector capabilities like the ability to unplug without soldering or support for hot plugging. Choose connectors designed specifically for debug access if these features are important.
Review Connector Reliability and Operating Lifetime
For Cortex devices used in high-reliability or safety-critical applications, connector reliability is crucial. Military-grade connectors like Harwin’s CombiTac series offer guaranteed lifetime mating cycles.
Environmental resistance is also important – connectors must retain stable contact resistance across temperature changes, shock/vibration, humidity etc. Choose connectors rated for automotive, aerospace or other ruggedness needs.
Select Reputable, Specialized Manufacturers
Specialized manufacturers like Tag-Connect, Samtec, and Picroft focus exclusively on debug connector products for ARM chips. Their connectors are designed keeping in mind Cortex developers’ specific needs.
Additionally, reputed brands assure strict quality control and superior materials. This results in reliable connectors proven through years of use in debug applications.
Example Debug Connector Selection
As an example, let’s say we need to select a connector for debugging a battery-powered IoT sensor board using NXP’s Cortex-M4 microcontroller, which supports SWD. The board has a compact footprint and 1.27mm hole pitch.
Based on the above tips, we would choose a compact 6-pin debug connector with SWCLK, SWDIO, GND and VCC pins. A right-angle connector suits the space constraints better. The connector should have 30u” hard gold plating for durability across repeated plugging. Samtec’s FTSH-105-01-F-DV-K-TR offers an excellent fit based on these requirements.
By considering the target processor’s debug interface, board layout, pin needs, mechanical specs, electrical properties, and usage requirements, an optimal Cortex debug connector can be selected.
Conclusion
Choosing the right debug connector is a crucial part of enabling effective embedded development using ARM’s Cortex processors. This guide summarizes the key selection criteria ranging from debug interface type, pin count, and plating to board space, tool compatibility, and manufacturer reputation.
Thoroughly evaluating your specific project needs against available connector options using the tips provided will ensure you pick the best Cortex debug connector for your application.