Protected Register Access Control for Implantable Medical Devices
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Solution Overview
Problem
Implantable medical devices (IMDs) face challenges in preventing unintended modifications to configuration registers due to soft errors, hard faults, and software bugs, which can lead to incorrect operation or malfunction, and existing solutions like redundant registers and parity checks are either costly or insufficiently protective.
Innovation Solution
A circuit and method that utilize a bus controller with an address register, unlock register, and protected register enable unit to manage access to protected registers, requiring a valid unlock code for modifications and incorporating a parity register for additional protection, ensuring that only authorized changes can be made to configuration settings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If redundant register utility is used to guard against errant write operations and radiation-induced single event upsets, then reliability is improved, but device complexity and cost increase due to multiple additional logic components
Solution Approach 1:
The protection mechanism is segmented into distinct functional units: an unlock register for authentication, a protected register for data storage, a comparator for validation, and a control unit for managing access. This segmentation allows each component to perform a specific function efficiently, reducing overall complexity compared to a monolithic redundant register system while maintaining reliability through distributed validation logic.
Solution Approach 2:
The system performs preliminary authentication by requiring a valid unlock code to be loaded into the unlock register before any write operations to the protected register are permitted. The comparator pre-validates the unlock code against the protected register contents, and only after this preliminary validation does the control unit enable write access. This preliminary action prevents unauthorized modifications before they can occur.
2Device complexity
If simple parity check with a single extra storage bit is used, then device complexity is reduced, but reliability is insufficient because a pair or even number of bits may change to incorrect states
Solution Approach 1:
The control unit acts as an intermediary that manages the relationship between the unlock register, protected register, and write control. It coordinates the validation process by receiving the unlock code, directing the comparator to validate it, and based on the validation result, either enabling or blocking write operations. This intermediary coordination ensures that even with minimal additional storage bits, the system achieves robust protection against multiple bit changes.
Solution Approach 2:
The comparator provides feedback about the validity of the unlock code to the control unit. When the unlock code matches the protected register contents, the comparator outputs a valid signal that feeds back to the control unit, which then enables write access. When there is a mismatch, the feedback signal blocks write operations. This feedback mechanism ensures continuous validation and prevents unauthorized or erroneous writes without requiring extensive additional storage.
3Reliability
If unlock code validation is performed for each write operation to protected register, then reliability is improved by preventing unauthorized modifications, but ease of operation decreases due to additional access steps
Solution Approach 1:
The protected register serves multiple functions: it stores configuration data, contains the authentication key for unlock validation, and controls access permissions. The control unit also performs multiple roles: managing write operations, validating unlock codes, and enabling or blocking access based on validation results. This multi-functionality reduces the need for separate dedicated components, simplifying the overall operation while maintaining security.
Solution Approach 2:
The protected register essentially validates its own access by storing the authentication key within itself. The unlock code is derived from or matches contents of the protected register, creating a self-contained authentication mechanism. This self-service approach eliminates the need for external authentication storage or complex multi-component validation systems, making the operation more straightforward while maintaining strong security.
Data Source
AI summary
Circuits, devices and methods are provided to manage modifications to protected registers within an implantable medical device (IMD). The circuit comprises a bus controller that includes an address register, an unlock register and a protected register (PR) enable unit. The PR enable unit sets a protect enable signal to an access state based on content loaded into the unlock register. A peripheral block includes a protected register that retains content for operating the IMD. The peripheral block includes a register access input to receive the protected enable signal. A PR write control unit is provided to enable an attempted write of the content from a data interface to the protected register when the protected enable signal has an access state.