Secure USB Time-Stamp for Data-Logging

Posted on Feb 7, 2014

This project implements a USB device which provides a real-time clock for the purpose of time-stamping events in an non-networked embedded computer environment. For embedded applications where a periodic time-stamp is required (such as entry-system logs, configuration audit logs, etc. ) it is necessary to have a fairly accurate real-time clock (bet

Secure USB Time-Stamp for Data-Logging
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ter than that typically provided by a PC`s motherboard) to generate time-stamps in logging and audit trails. Furthermore, it is preferable to have a method of confirming that the log/audit files have not been tampered with in anyway. The secure USB time-stamp device solves many of these issues in a very small form factor using minimal components. The device logs the time at which the clock was last updated and (if requested by the USB host) the time when it last served a time-stamp. This allows the host application to examine it`s own logs and compare them to the time values stored by the device (to detect when logs have been altered). The device stores the last time setting and the last logged time-stamp in the PIC`s non-volatile EEPROM memory which can hold the data for up to 40 years without battery backup. The SUTS device is powered by a PIC18F2550 microcontroller which provides an on-board USB interface as well as the required SPI serial interface to the DS1302 real-time clock chip. To prevent loss of time setting when powered off or disconnected the DS1302 is backed up by a 0. 047F super-capacitor. A super-capacitor was chosen (rather than a lithium cell) since it is harder to remove (preventing a casual or deliberate reset of the time), charges very fast (less than 5 minutes from empty to fully-charged) and is capable of powering the RTC for around 90 hours without losing time. Furthermore, the super-cap has a...

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