DFT DRC and script automation

Introduction

GOF ECO provides a streamlined solution for designers to implement functional changes in digital designs without compromising DFT functionality. This article highlights key features of DFT-friendly ECO and the DFT Design Rule Checking (DRC) capabilities integrated into GOF ECO.

A prime example of GOF DFT DRC's utility is its ability to detect issues like those in manual ECOs, such as mistakenly tying the scan shift enable pin to zero instead of connecting it to the DFT scan shift signal. GOF's DFT DRC catches such errors, as illustrated in Figure 1.

Figure 1: DFT DRC example

DFT-Friendly ECO in GOF ECO

Preserving DFT Logic: GOF ECO ensures that DFT logic remains unaffected during functional ECOs. It sets DFT logic to an inactive mode and excludes it from the ECO process to maintain its functionality. This is achieved using APIs like `set_pin_constant`, `set_net_constant`, `set_ignore_output`, and `set_ignore_pin`.

Maintaining Scan Chain Integrity: GOF ECO ensures that the scan chain remains intact after functional ECOs. If an ECO involves inserting functional logic between back-to-back flops, GOF ECO can either:

• Change the drain flop to a scan type flop with scan_in and scan_enable pins (Solution 1).
• Insert a MUX before the D input of the drain flop, with the selection signal controlled by the scan_enable signal (Solution 2).

Managing Clock and Reset Signals: GOF ECO ensures that clock and reset signals are controllable and do not cause glitches during test mode. It checks that these signals are defined as DFT clocks, controllable in DFT mode, and do not have multiple active paths that may cause glitches.

Handling DFT Test Points: GOF ECO ensures that DFT test points remain functional and accessible for testing after functional ECOs. It preserves DFT test point accessibility, constrains DFT test point signals, and checks for any issues with DFT test points during DFT DRC.

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DFT Design Rule Checking in GOF ECO

GOF ECO includes a DFT Design Rule Checker that helps ensure the integrity of DFT logic after an ECO. The DFT DRC checks for various errors, such as:

• Broken scan chains (ERROR_MULTI_PATHS)
• Scan chains ending with constants (ERROR_END_CONST)
• Scan chains ending with non-flops or non-EDT logic (ERROR_END_AT_INST)
• DFT test points being driven by non-controllable signals (ERROR_DFT_TESTPOINT_DRIVEN)
• DFT test points having multiple active paths that may cause glitches (ERROR_DFT_TESTPOINT_GLITCH)
• Flops driven by clocks not defined as DFT clocks (ERROR_CLOCK_UNDEFINED)
• Flops with reset pins having multiple active paths that may cause glitches (ERROR_RESET_GLITCH, ERROR_SET_GLITCH)

DFT DRC script generated by AI technical support

Using the NanDigits AI-powered technical support platform, designers can automatically generate a DFT DRC script template. This template can then be customized as needed by the user.

Figure 2: DFT DRC script automation

Conclusion

GOF ECO's DFT-friendly features and robust DFT DRC ensure that designers can efficiently implement functional changes while preserving essential DFT functionalities. By maintaining scan chain integrity, managing clock and reset signals, and protecting DFT test points, GOF ECO minimizes the risk of DFT-related issues. Additionally, the AI-powered DRC script automation provides further convenience, enabling faster and more reliable ECO processes, leading to improved design testability and reliability.