Boundary scan, also known as JTAG, is a powerful testing methodology for printed circuit boards (PCBs). This article explores boundary scan architecture, applications, and the essential role of Boundary Scan Test Tools in ensuring product quality and reducing development costs.
A Brief History of Boundary Scan and JTAG
The increasing complexity of PCBs in the 1980s led to the formation of the Joint Test Action Group (JTAG), which developed a specification for boundary scan testing. Standardized as IEEE Std. 1149.1-1990, and later revised as 1149.1a in 1993, this standard revolutionized PCB testing. Boundary scan enables testing of interconnects between integrated circuits without physical probes, significantly improving testability.
Boundary Scan Architecture and the TAP Interface
The IEEE 1149.1 standard defines test logic within integrated circuits, enabling features like chain integrity testing, interconnection testing, and in-system programming. Central to this architecture is the Test Access Port (TAP), a standardized interface for accessing and controlling the boundary scan logic. The TAP controller manages the flow of test data and instructions through dedicated signals like Test Clock (TCK), Test Mode Select (TMS), Test Data In (TDI), and Test Data Out (TDO).
Essential Boundary Scan Test Tools
Effective boundary scan implementation requires specialized boundary scan test tools. These tools typically consist of two main components:
- Test Program Generators (TPGs): TPGs analyze the board’s netlist and BSDL files to automatically generate test patterns for interconnect testing. Advanced TPGs, like Corelis’ ScanExpress TPG, can achieve high net coverage even without design optimization.
- Test Execution Tools: These tools execute the generated test patterns, compare results, and provide diagnostics for fault isolation. Corelis’ ScanExpress Runner is an example, offering features like test plan management, in-system programming, and detailed failure analysis.
Applications of Boundary Scan Test Tools
Boundary scan test tools find applications across the entire product lifecycle:
- Product Development: Early design analysis, prototype debugging, and faster fault isolation.
- Production Test: Replacing or supplementing traditional in-circuit testers (ICTs), enabling faster test development and execution, and reducing fixture costs.
- Field Service and Installation: Remote diagnostics, firmware updates, and system maintenance. JTAG Emulation Test (JET) extends boundary scan capabilities to functional testing at full speed, enhancing fault coverage.
Design Considerations for Boundary Scan
Optimizing designs for boundary scan testability involves following simple guidelines:
- Strategically placing programmable devices in the scan chain.
- Using 1149.1-compliant components.
- Properly buffering TAP signals.
- Ensuring access to non-boundary scan components for testing.
Conclusion
Boundary scan test tools are indispensable for modern electronics manufacturing. By leveraging the IEEE 1149.1 standard, these tools provide comprehensive testing solutions, significantly reducing development and production costs while improving product quality. They are crucial for testing complex PCBs with limited physical access, ensuring reliable interconnections, and enabling efficient in-system programming. From design validation to field service, boundary scan remains a vital technology for the electronics industry.
References
- IEEE Std 1149.1-1990 – Test Access Port and JTAG Architecture
- IEEE Std 1149.1-1994b – Supplement to IEEE Std 1149.1-1990 (http://www.ieee.com/)
