Fault-Tolerant Scannable Glitch Latch for Stuck-At Fault Isolation
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Solution Overview
Problem
Standard scan chains in machines like central processing units are susceptible to stuck-at faults, which prevent proper debug, reset, and repair by propagating faults through the chain, impacting the ability to initialize the machine to a known state.
Innovation Solution
A fault-tolerant scannable glitch latch is introduced, utilizing a scan shift enable signal to prevent stuck-at zero faults from propagating by providing a parallel path to ground and controlling switches to ensure the state node is not driven high, allowing proper reset and repair even with stuck-at faults.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If scan chains are used to reset the initial state of a machine, then the machine can be initialized to a known state, but stuck-at faults in the array may be propagated onto the scan chain and prevent debug, repairability, or resetting
Solution Approach 1:
An intermediary circuit is introduced between the array and scan chain consisting of two switches controlled by scan shift enable signals. This intermediary prevents direct propagation of stuck-at faults from the array to the scan chain while allowing legitimate data to pass through during scan operations
Solution Approach 2:
The data path is segmented into multiple controllable sections using switches. The first switch segments the path from the array output to the scan chain input, and the second switch segments the parallel path to ground, allowing independent control of fault propagation paths
2Reliability
If a stuck-at zero fault occurs on a data input line, then voltage may be driven to a state node or pulled high during scan chain operation, but the scan shift enable signal prevents this by controlling switches
Solution Approach 1:
The scan shift enable signal activates switches in advance to prevent the harmful effect of stuck-at zero faults before they can occur. The first switch is positioned to block voltage driving to the state node, and the second switch is positioned to block pulling the state node high, creating preliminary protection against fault effects
3Ease of operation
If reset signals are distributed to each state element to clear current state, then each state element can be reset to a known state, but this requires complex distribution of reset signals to each element
Solution Approach 1:
The scan chain is given multi-functionality to serve both as a debug/repair interface and as a reset mechanism. By controlling the switches with scan shift enable signals, the same scan chain infrastructure performs multiple functions including initialization, debugging, and repair operations
Solution Approach 2:
The reset function is merged with the scan chain operation. Instead of separate reset signal distribution networks, the scan chain itself is used to initialize state elements by controlling the switches to allow or block data flow during scan operations
Data Source
AI summary
A fault tolerant scannable glitch latch for use with scan chains that enable reset, debug and repairability of machines and parts is described. A scan shift enable signal controls a switch such that a stuck-at zero fault on a data input line is prevented from driving voltage to a state node or pulling the state node high during a scan chain operation. Propagation of the stuck-at zero fault is therefore eliminated. The scan shift enable signal also controls a switch that enables a parallel path to ground for the scan data and state node which would otherwise have been driven high due to the stuck-at zero fault.