Designing for Testability: A Hallmark of Experienced Engineering
One habit that has served me well as an embedded engineer has always been to design both the hardware and the firmware to be testable. You want to make things accessible. It makes systems easier to debug, easier to validate, and easier to maintain.
Many engineers begin a design by selecting a microcontroller and evaluating the manufacturer’s development board. The problem is that evaluation boards are designed for convenience and rarely have the same space constraints as production products. Engineers often copy the programming and debugging connector directly from the evaluation board without realizing that they could use something more space efficient, without loss of functionality.
An experienced engineer asks “How am I going to prove this works?”
It’s a question that influences many design decisions. It may mean bringing out I²C, SPI, or other signals so they can be monitored with a logic analyser. It may mean exposing test points for validation. It may mean providing access to diagnostic information that will be needed months later when a problem appears.
As systems become more complex, the ability to observe what is happening inside a design becomes increasingly important. Whether you are debugging firmware, validating hardware, or investigating a field failure, the ability to access to information tells you what the system is doing.
Acting on that principle has become harder as boards have become smaller and smaller. Sometimes the native program/debug connector is as large as, or even bigger than, the board itself. Oftentimes engineers resort to cutting cables in half and splicing their own custom programming connections because the standard solutions are bulky, expensive, or simply unsuitable for the available space.
The challenge of preserving access to programming and debug interfaces on increasingly tiny boards ultimately led me to develop what became the Plug-of-Nails™ system. The aim of our cables was not simply to shrink the connector but to use a tiny footprint that would still preserve full programming, testing, and debugging access, all while consuming as little PCB space as possible. Our designs (often as small as a 0805 resistor) allow the connector to be placed right at the microcontroller, reducing trace lengths, improving signal integrity, minimising exposure to electrical noise, and making design validation easier throughout development and production. The ECV3 Edge Connect™ series saves even more space by literally using the edge of the circuit board.
Another trend I have observed is that debug interfaces rarely become simpler. Tool vendors continue to add functionality such as trace, virtual serial ports, power measurement, and other capabilities. ST’s STDC14 connector is one example, expanding the traditional 10-pin Arm® Cortex® Debug Connector by four additional pins, a 40% increase in connector size.
As new functions are added, preserving board space becomes increasingly challenging. Tag-Connect’s upcoming 8-pin solution is in many ways, an answer to this. It expands our traditional 6-pin Cortex® implementations to accommodate the virtual COM port functionality now being adopted by many tool vendors, while maintaining a substantially smaller footprint than many other traditional and new debug connectors. Our latest adapter is a reflection of that conviction: eliminating the additional pins while keeping full test functionality.
Programming, debug, and test access should be considered part of the system architecture from the beginning. Too often, these interfaces become an afterthought, only to discover later that the signals needed to diagnose a problem are buried within internal PCB layers or were omitted entirely. By then, what could have been a simple investigation becomes an expensive, time-consuming nightmare.
The most robust systems are often the most transparent. If you can observe internal state, inspect configuration, and collect meaningful diagnostic information, most problems become straightforward to understand.