What is IJTAG: A Standard for Accessing Embedded Instruments

Introduction

If you are a chip designer or a test engineer, you may have heard of JTAG, a standard that provides a way to access and control the chip boundary for validation, test and debug applications. But have you heard of IJTAG, a standard that extends JTAG to access and control the embedded instruments within the chip? In this blog post, we will introduce what IJTAG is, how it differs from JTAG, and why it is useful for chip design and test.

What is IJTAG?

IJTAG stands for Internal JTAG. It is a standard that streamlines the use of instruments that have been embedded in chips. It facilitates the deployment of these embedded instruments in a wider array of chip, board and system level validation, test and debug applications. Chip designers find IJTAG useful during design verification when it will be used in conjunction with a simulator or emulator. IJTAG is deployed in the ATE test environment where it becomes part of chip test, chip debug/diagnostics and chip characterization.

IJTAG defines an Instrument Access Network (IAN) that can access a set of registers called Instrument Data Registers (IDR) inside the chip. The IAN is composed of segments that connect the instruments to each other and to the JTAG port. The segments can be reconfigured to form different scan paths for different purposes. The IAN also supports hierarchical access, which allows accessing instruments at different levels of abstraction.

IJTAG also defines a standard format for describing the instruments and their features, called IEEE 1687 Instrument Connectivity Language (ICL). The ICL file can be used by software tools to automatically generate test patterns, retarget patterns across different levels of hierarchy, and perform fault diagnosis and calibration.

How does IJTAG differ from JTAG?

JTAG stands for Joint Test Action Group. It is a standard that provides the chip instruction, data and state logic necessary to support validation, test and debug applications. JTAG defines a Test Access Port (TAP) and a controller that can access a chain of registers called Test Data Registers (TDR) on the chip boundary.

JTAG has been widely used for testing printed-circuit boards (PCBs) and integrated circuits (ICs) since at least 1990. JTAG also has been used in a variety of ad-hoc manners to access embedded instruments such as built-in self-test (BIST) engines, complex I/O characterization, and embedded timing instruments.

IJTAG differs from JTAG in the following ways:

• IJTAG uses the JTAG infrastructure within silicon to provide physical access to embedded instruments. IJTAG defines an Instrument Access Network (IAN) that can access a set of registers called Instrument Data Registers (IDR) inside the chip.
• IJTAG was developed to supplement JTAG with the flexibility, re-configurability and functional features that would make working with the myriad of embedded instruments easier and more effective. IJTAG also enables plug-and-play features for instruments, which simplifies the integration and reuse of IP blocks.
• JTAG is compatible with IJTAG and can be modeled within the IJTAG standard. In fact, the JTAG port is used as the IJTAG primary top-level device interface. Thus, IJTAG access operates through the JTAG port to control a variety of embedded test features and other instruments.

Why is IJTAG useful?

IJTAG improves testability over traditional methods by providing a standardized way to access and control embedded instruments within a chip. IJTAG allows for reconfigurable scan paths, reduced access time, and pattern retargeting for different levels of hierarchy. IJTAG also enables plug-and-play features for instruments, which simplifies the integration and reuse of IP blocks.

IJTAG can leverage the existing JTAG infrastructure to access the embedded instruments, but also provides more flexibility and functionality than JTAG. IJTAG can support various validation, test and debug applications throughout the chip and system life cycle. This portends a significant cost reduction for design and test engineers, along with greatly enhanced fault coverage and diagnostics.

Embedded instruments are logic structures within a device whose purpose is Design for Test (DFT), Design-for-Debug (DFD), Design-for-Yield (DFY), Test, Debug, Characterization, Monitoring, or similar activities. Embedded instruments can perform various functions such as generating test patterns, measuring signals, calibrating I/Os, monitoring performance, etc.

Some examples of embedded instruments are:

• BIST (Built-In Self-Test) engines: These are instruments that can test the functionality of a device or a circuit without external stimuli or equipment.
• Complex I/O Characterization and Calibration: These are instruments that can measure and adjust the parameters of high-speed serial I/Os such as voltage, timing, jitter, etc.
• Embedded Timing Instrumentation: These are instruments that can measure the timing characteristics of clocks and signals within a device or a circuit.
• Bit Error Rate Test (BERT) engines: These are instruments that can measure the error rate of digital communication channels by comparing transmitted and received data patterns.
• Margining engines: These are instruments that can vary the operating conditions of a device or a circuit such as voltage, frequency, temperature, etc. to determine the performance margins and reliability.
• Memory BIST: These are instruments that can test the functionality and integrity of memory devices such as SRAM, DRAM, Flash, etc.
• Random Pattern Generators: These are instruments that can generate random or pseudo-random data patterns for testing purposes.
• Logic Analyzers: These are instruments that can capture and analyze the logic states of multiple signals within a device or a circuit.

Conclusion

IJTAG is a standard that streamlines the use of embedded instruments within chips. It provides a way to access and control these instruments using the JTAG port. It also provides a way to describe these instruments using a standard language. IJTAG is useful for chip design and test, as it improves testability, reduces cost, and enhances quality. IJTAG is compatible with JTAG, but also provides more flexibility and functionality than JTAG. IJTAG is a powerful tool for working with embedded instruments in chips.