Boundary Scan in Design for Test: How It Works and Why It’s Important

With the increase in the complexity of integrated circuits (ICs), it is essential that these work properly with reduced time and cost for their testing. One of the developed methodologies to meet this challenge is Boundary Scan Test in VLSI design. Boundary Scan is an excellent testing and diagnosis methodology used for testing complex digital systems both at the manufacturing stage and in the field, so fault detection and isolation are much easier.

This article will explain how to test boundary scan functions, its significance in the boundary scan in DFT (design for test) process, and its role within modern IC design. The architecture, the logic behind it, and boundary scan cell types used in this method are described, along with the advantages to the VLSI designer and the company.

What is Boundary Scan?

Boundary Scan is an approach to testing interconnections between ICs in a system. It provides a way to do chip and board testing without physical test probes. Rather than being dependent upon traditional in-circuit testing approaches, boundary scanning involves extra logic added to the design to test nodes internally and the external I/O pins controlled by software.

At its essence, Boundary Scan Architecture adds extra flip-flops along the edges of a digital integrated circuit. However, boundary scan cells, which consist of such a feature, can capture input/output signals from across a digital IC and shift data into or out of the scan chain to drive it to internal circuitry.

How Does Boundary Scan Work?

The boundary scan architecture comprises a string of boundary scan cells inserted along each input/output (I/O) pin in the chip. In a scan chain, this string acts as a shift register that can reach every signal state of any I/O pin. The basic mechanism is as follows:

1. Test Access Port (TAP) and Controller: The test information is fed into the chip through a specific interface known as the TAP controller. This controller controls the passing of test information both towards the entry and also out of the boundary scan cells.
2. Shift Register Operation: Boundary scan cells are placed in a serial shift register. During the test, cells may be configured to shift data either into or out of this chain. This would allow data to be captured or applied to the I/O pins.
3. Test Execution: The boundary scan test has different test modes: sample, preload, and capture. This means that engineers can view the chip’s internal signals, the stimulus applied, and the responses without having to probe the chip in physical action.
4. Fault Isolation: Now, with the data shifted through the boundary scan cells, the engineer can detect and diagnose open circuits, short circuits, and stuck-at faults, a very handy capability for systems with many devices, like modern PCBs and chips.

Key Components of Boundary Scan Architecture

There are the following critical components that comprise this boundary scan architecture, which enables this method to work efficiently:

• Boundary Scan Cells: These cells capture and regulate the data at every I/O pin in the boundary scan process. There are various kinds of boundary scan cells. They include input cells, output cells, and bidirectional cells designed for specific functions.
• Scan Chain: These are a sequence of boundary-scanned cells chained together to form a shift register that permits data to be shifted through them during the actual test so that those I/O pins can be seen and controlled.
• Test Access Port (TAP): In this case, the TAP controller interacts with boundary scan in DFT logic to manage the flow of data and permit test modes.
• Instruction Register: The instruction register executes different test modes, including bypass, sample, and boundary scan tests in VLSI modes.

Boundary Scan Logic and the Scan Chain Tutorial

To understand the concept of boundary scan logic, it is most important to have a good comprehension of how a scan chain works. A scan chain consists of several flip-flops cascaded together to capture the values of input pins or apply data on output pins. Engineers would use this chain to serially shift test data into and through the flip-flops, thereby making testing easy in each circuit.

It makes the test procedure provide access to internal signals otherwise inaccessible, thereby making debugging and testing of VLSI designs more efficient through scan chains. It is necessary for boundary scan testing because faults on the circuit board can be identified without physical probing, while the effectiveness of DFT strategies in integrated circuits is enhanced. In the context of a scan chain tutorial, one would learn about very common concepts, including:

• Scan In and Scan Out: This simply refers to the shifting of test data into the boundary scan cells via the “Scan In” pin and out via the “Scan Out” pin.
• Capture and Update: The capture mechanism in boundary scan testing is used to capture the values at the I/O pins, while the update mechanism in the same boundary scan testing is used for the update of the output pins with test data.
• Test Modes: Examples of test modes, such as EXTEST – test of external I/O or INTEST, which tests the internal logic.

Conclusion: Why Boundary Scan Matters at Takshila VLSI

Boundary scan plays an important role in modern VLSI design and testing, which represents a scalable and efficient way of getting surety in the working conditions of highly complex chips and boards. To become good engineers, they must gain knowledge about VLSI by mastering boundary scan architecture and logic. The vast training in Takshila VLSI is on boundary scan in DFT, boundary scan testing, and implementation of scan chains.

Learn the art of implementing boundary scan tests with our advanced VLSI training courses and also experience the world-class learning environment with career growth and industry placement support. Check out our courses and join the leading VLSI training institute at Takshila VLSI to take your skills to the next level.