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HCS515 Datasheet(PDF) 7 Page - Microchip Technology |
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HCS515 Datasheet(HTML) 7 Page - Microchip Technology |
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7 / 38 page  © 2011 Microchip Technology Inc. DS40183E-page 7 HCS515 3.4 Synchronization with Decoder (Evaluating the Counter) The KEELOQ technology patent scope includes a sophisticated synchronization technique that does not require the calculation and storage of future codes. The technique securely blocks invalid transmissions while providing transparent resynchronization to transmitters inadvertently activated away from the receiver. Figure 3-3 shows a 3-partition, rotating synchronization window. The size of each window is optional but the technique is fundamental. Each time a transmission is authenticated, the intended function is executed and the transmission's synchronization counter value is stored in EEPROM. From the currently stored counter value there is an initial "Single Operation" forward win- dow of 16 codes. If the difference between a received synchronization counter and the last stored counter is within 16, the intended function will be executed on the single button press and the new synchronization coun- ter will be stored. Storing the new synchronization counter value effectively rotates the entire synchroniza- tion window. A "Double Operation" (resynchronization) window fur- ther exists from the Single Operation window up to 32K codes forward of the currently stored counter value. It is referred to as "Double Operation" because a trans- mission with synchronization counter value in this win- dow will require an additional, sequential counter transmission prior to executing the intended function. Upon receiving the sequential transmission the decoder executes the intended function and stores the synchronization counter value. This resynchronization occurs transparently to the user as it is human nature to press the button a second time if the first was unsuc- cessful. The third window is a "Blocked Window" ranging from the double operation window to the currently stored synchronization counter value. Any transmission with synchronization counter value within this window will be ignored. This window excludes previously used, perhaps code-grabbed transmissions from accessing the system. FIGURE 3-3: SYNCHRONIZATION WINDOW Blocked Entire Window rotates to eliminate use of previously used codes Single Operation Window Window (32K Codes) (16 Codes) Double Operation (resynchronization) Window (32K Codes) Stored Synchronization Counter Value |
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