Importance

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Abstract

A real time clock (RTC) is described several timekeeping dependability and timekeeping security attributes. The RTC may have several registers for storing values, at least one of which stores a value which is safeguarded. For example, a 'TrustQualityState' register stores a 'TrustQualityState' value which is dependent upon a timekeeping accuracy of the RTC. The 'TrustQualityState' value may also be dependent upon timekeeping stability, reliability, and/or security of the RTC (e.g., a tamper resistance of the RTC). The RTC includes an access unit coupled between the 'TrustQualityState' register and a bus used to access the 'TrustQualityState' register. The access unit controls access to the 'TrustQualityState' register in order to safeguard the 'TrustQualityState' value. The access unit receives read and write commands directed to 'TrustQualityState' register via the bus. The access unit determines if a source of a received read or write command directed to the register is authorized to access the register, and provides access to the register only if the source is authorized to access the register. The 'TrustQualityState' value may expire a predetermined period of time after issue. In this case, the access unit may also include a counter configured to issue a signal after the predetermined period of time. When the access unit receives the signal from the counter, the access unit may store a default value of '0' in the 'TrustQualityState' register.

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First Claim