DFT Implementation Guidelines for Multi-Clock, Multi-Power SoC Designs

As modern System-on-Chip devices grow in complexity, Design-for-Testability has become a critical design discipline — not a post-design afterthought. Today's automotive, AI, networking, industrial, and IoT SoCs contain millions of sequential elements distributed across multiple clock domains, voltage islands, embedded memories, processors, accelerators, and safety-critical subsystems.

Successful DFT architecture must ensure high manufacturing fault coverage while minimising silicon overhead, test time, and tester memory requirements. Achieving this requires careful planning of scan architecture, memory BIST infrastructure, test compression, at-speed testing, and safety diagnostics from the earliest stages of SoC development.

This article discusses practical guidelines for implementing DFT in multi-clock, multi-power SoCs and highlights key considerations for scan insertion, scan chain planning, memory BIST deployment, test compression, and at-speed test methodologies.

DFT Challenges in Multi-Clock Multi-Power SoCs

Modern SoCs rarely operate from a single clock source. A typical design may include processor clocks, AXI interconnect clocks, peripheral clocks, security clocks, sensor clocks, and low-power always-on clocks. In addition, multiple voltage domains are often introduced to reduce power consumption.

From a DFT perspective, each clock and power domain introduces additional complexity. During test mode, scan chains must remain controllable and observable while respecting clock domain boundaries and power isolation requirements.

For example, in a 3.5-million-flop automotive SoC containing six clock domains, scan planning is typically performed independently for each clock domain before integrating the entire scan architecture.

Scan Planning and Scan Chain Architecture

Scan planning begins by grouping all scan-capable flip-flops according to their functional clock domains. Consider the following example:

The next step is to determine the desired scan chain depth. A widely adopted guideline is 2,000 flops per scan chain — an excellent balance between routing complexity and test time. The number of chains is estimated using:

A domain containing 96K flops would require approximately 48 internal scan chains, which can be driven with 8 chains running in parallel. The objective is to keep all chains approximately equal in length — balanced scan chains reduce shift cycles and improve compression efficiency.

Determining the Number of Parallel Scan Channels

Internal scan chains are eventually connected to a much smaller number of external tester channels through a compression architecture. For example:

• Internal scan chains: 48
• External scan channels on ATE: 6

Without compression, all 48 scan chains would need to be shifted directly from the tester. With compression, only 6 channels are driven by the tester while on-chip decompression hardware expands the stimulus across all internal chains. The choice of external channels depends on available tester pins, desired test time, compression ratio, and area overhead. Automotive and networking SoCs commonly use 4 to 16 external scan channels while driving thousands of internal scan chains.

Scan Stitching Methodology

After scan insertion, scan stitching connects scan-enabled flip-flops into complete scan chains. The stitching process follows strict rules:

• All flip-flops in a chain must belong to the same clock domain.
• Power-domain boundaries should not be crossed.
• Safety-critical lockstep processors should maintain separate scan visibility.
• Non-scan elements must be bypassed.
• Clock-generation state machines and other test-sensitive logic may require exclusion from scan chains.

A typical scan stitching flow:

1. Identify scannable flops.
2. Group flops by clock domain.
3. Group flops by power domain.
4. Balance chain lengths (exact balance is not always achievable).
5. Create scan chain connectivity.
6. Verify scan DRC compliance.
7. Insert compression logic.

Proper scan stitching is one of the most important factors affecting ATPG quality and final fault coverage.

Importance of Scan Chain Depth in Test Time Optimisation

Test time is largely determined by scan chain length. Consider two scenarios:

The second implementation reduces scan shift time by almost ten times. However, increasing chain count also increases routing congestion and compression hardware. The optimum solution generally falls between 500 and 2,000 flops per chain depending on SoC size. For large SoCs, approximately 2,000 flops per chain remains an industry-proven guideline.

Memory BIST Implementation Methodology

Embedded memories frequently occupy more than 60% of modern SoC area. Traditional scan testing is inefficient for memories because of the extremely large number of storage elements. Memory Built-In Self-Test (MBIST) solves this by incorporating dedicated test controllers that execute memory test algorithms internally.

A memory BIST architecture consists of:

• MBIST controller
• Address generator
• Data pattern generator
• Comparator
• Signature register
• Test access interface

The controller automatically applies test patterns and evaluates responses without requiring large ATPG pattern sets.

Memory Clustering Strategy

One of the most important MBIST decisions is whether memories should share a controller or receive dedicated controllers. A practical guideline:

For example, six radar FIFO memories of 8 KB each may share a single MBIST controller, tested sequentially under one controller to significantly reduce silicon overhead. On the other hand, L1 caches, L2 caches, tightly coupled memories, and shared SRAMs typically justify dedicated MBIST controllers due to their size and performance importance. The clustering approach balances test efficiency against area overhead.

Memory Test Algorithms

The most widely used MBIST algorithm is March C−. It effectively detects stuck-at faults, transition faults, address decoder faults, and coupling faults through ordered read and write operations performed in ascending and descending address order.

For high-reliability automotive and aerospace products, retention tests may be added to detect data retention failures. Cache memories often combine March algorithms with retention testing for improved coverage.

Boot RAM Self-Test During Initialisation

Safety-critical processor SoCs frequently execute memory self-tests during power-on initialisation. Examples include Boot RAM, Safety RAM, Tightly Coupled Memory (TCM), Security processor memory, and Hardware Security Module memory.

At power-up the sequence is:

1. Processor remains in reset.
2. MBIST engine starts automatically.
3. Memory is tested using March algorithms.
4. Pass/fail signature is generated.
5. Boot sequence proceeds only after successful completion.

This approach enables Power-On Self-Test (POST) compliance and helps satisfy functional safety requirements. For ASIL-D designs, automatic startup memory diagnostics are often mandatory.

Signature Capture and MISR-Based Analysis

Rather than storing every response bit, DFT architectures typically compact responses into a signature using a Multiple Input Signature Register (MISR). At the end of testing, the captured signature is compared against a golden reference. A match indicates a pass; any difference indicates a fault. Signature analysis dramatically reduces tester memory requirements and simplifies production testing.

Test Compression for Test Time Reduction

Without compression, large SoCs may require millions of ATPG patterns. Compression logic reduces both input stimulus volume and output response volume. A typical architecture contains two key blocks:

• Decompressor — generates thousands of internal scan stimuli from a few external channels.
• Compactor — compresses responses into a smaller observable data stream.

As an example, 8 tester channels can efficiently control 3,500 internal scan chains. Compression ratios exceeding 100× are routinely achieved in modern SoCs, dramatically reducing ATE memory usage and manufacturing test cost.

At-Speed Testing and Its Importance

Stuck-at testing alone cannot detect timing-related defects. Modern process technologies require transition fault testing at actual operating frequencies to verify slow-to-rise faults, slow-to-fall faults, and path delay defects.

The standard methodology applies a launch pulse followed by a capture pulse at functional frequency. This creates realistic switching activity and exposes timing defects that are invisible during low-speed testing.

Achieving At-Speed Test Using OCC

External testers typically cannot generate the high-frequency clocks required by modern SoCs — a processor operating at 200 MHz or several GHz cannot be tested directly through tester pins. An On-Chip Clock Controller (OCC) solves this problem.

During scan shift, the tester supplies a slow scan clock. During capture, the OCC generates one or two functional-speed pulses derived from the on-chip PLL. The OCC typically supports four modes:

• Functional mode
• Scan-shift mode
• Stuck-at capture mode
• At-speed capture mode

Each clock domain receives its own OCC to ensure accurate frequency testing, enabling true at-speed validation without requiring extremely high-speed ATE resources.

Best Practices for DFT Signoff

• Begin DFT architectural planning early — alongside SoC architecture definition, not after RTL completion.
• Keep scan chains confined strictly to their clock and power domains.
• Balance chain lengths to minimise test time.
• Select Memory BIST controllers based on memory size and safety requirements.
• Ensure critical boot memories support automatic startup self-test.
• Adopt a compression architecture for all large SoCs to reduce ATPG pattern count and tester cost.
• Incorporate at-speed testing using OCC-generated functional clocks for high defect coverage in advanced process nodes.

When these principles are applied systematically, SoC teams achieve high manufacturing quality, lower production test cost, shorter test times, and improved field reliability.