Apparatus and method for sensitive circuit protection with set-scan testing
First Claim
1. An electrical circuit having a plurality of subcircuits and a set-scan test input for testing said subcircuits in a set/scan test mode, some of said subcircuits being sensitive subcircuits protected from reverse engineering, wherein the improvement comprises:
- selectable means, coupled to at least some of said sensitive circuits, for inhibiting set/scan test access to said at least some of said sensitive subcircuits, andmeans for actuating said selectable means, coupled between said subcircuits and said selectable means, to inhibit set/scan access to said sensitive subcircuits while permitting set/scan access to the other subcircuits.
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Accused Products
Abstract
A set/scan test capability is provided for a circuit that includes sensitive subcircuits, but that can be latched out to prevent reverse engineering the sensitive elements. A mechanism to inhibit set/scan test access to at least some of the sensitive subcircuits is selectively actuated by a control circuit to override a normal set/scan test and inhibit set/scan access to the sensitive subcircuits. Various implementations are possible, such as fusible-link PROMs for irreversibly inhibiting set/scan access to the sensitive subcircuits after an initial non-inhibited test period, the use of encryption codes to enable repeated set/scan access to the sensitive subcircuits, and an erasable/reprogrammable mechanism for inhibiting set/scan access to programmed sets of subcircuits.
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Citations
27 Claims
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1. An electrical circuit having a plurality of subcircuits and a set-scan test input for testing said subcircuits in a set/scan test mode, some of said subcircuits being sensitive subcircuits protected from reverse engineering, wherein the improvement comprises:
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selectable means, coupled to at least some of said sensitive circuits, for inhibiting set/scan test access to said at least some of said sensitive subcircuits, and means for actuating said selectable means, coupled between said subcircuits and said selectable means, to inhibit set/scan access to said sensitive subcircuits while permitting set/scan access to the other subcircuits. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
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14. An electrical circuit having a plurality of interconnected subcircuits, some of which are sensitive subcircuits protected from reverse engineering, a set/scan test input for testing said subcircuits in a set/scan test mode, respective bypass circuits for bypassing said subcircuits, respective switches for steering a set/scan test signal between a subcircuit and the bypass circuit for said subcircuit, and a set/scan control means for controlling the operation of said switches during a normal set/scan test, wherein the improvement comprises
access control means coupled between the set/scan control means and the switches for said sensitive subcircuits to control the operation of said switches and thereby the access of set/scan test signals to said sensitive subcircuits, and selectable means, coupled to said access control means, for operating said access control means to convert said control means from an initial state enabling the access of set/scan signals to said sensitive subcircuits, to a secure state in which said access control means operates the switches for said sensitive subcircuits to override said set/scan control means and inhibit the access of set/scan test signals to at least some of said sensitive subcircuits.
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23. A method of protecting an electric circuit that includes sensitive subcircuits protected from reverse engineering and a set/scan test capability from reverse engineering said sensitive subcircuits using the set/scan test capability, comprising the steps of:
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initially providing a set/scan test access to said sensitive subcircuits in said circuit; and thereafter inhibiting set/scan test access to said sensitive subcircuits. - View Dependent Claims (24, 25, 26, 27)
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Specification