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HCS515-P Datasheet(PDF) 3 Page - Microchip Technology |
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HCS515-P Datasheet(HTML) 3 Page - Microchip Technology |
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3 / 28 page  © 2002 Microchip Technology Inc. DS40183D-page 3 HCS515 FIGURE 1-1: CREATION AND STORAGE OF CRYPT KEY DURING PRODUCTION The 16-bit synchronization counter is the basis behind the transmitted code word changing for each transmis- sion; it increments each time a button is pressed. Due to the code hopping algorithm’s complexity, each incre- ment of the synchronization value results in greater than 50% of the bits changing in the transmitted code word. Figure 1-2 shows how the key values in EEPROM are used in the encoder. Once the encoder detects a button press, it reads the button inputs and updates the syn- chronization counter. The synchronization counter and crypt key are input to the encryption algorithm and the output is 32 bits of encrypted information. This data will change with every button press, its value appearing externally to ‘randomly hop around’, hence it is referred to as the hopping portion of the code word. The 32-bit hopping code is combined with the button information and serial number to form the code word transmitted to the receiver. The code word format is explained in greater detail in Section 7.2. A receiver may use any type of controller as a decoder, but it is typically a microcontroller with compatible firm- ware that allows the decoder to operate in conjunction with an HCS515 based transmitter. Section 3.0 provides detail on integrating the HCS515 into a sys- tem. A transmitter must first be ‘learned’ by the receiver before its use is allowed in the system. Learning includes calculating the transmitter’s appropriate crypt key, decrypting the received hopping code and storing the serial number, synchronization counter value and crypt key in EEPROM. In normal operation, each received message of valid format is evaluated. The serial number is used to deter- mine if it is from a learned transmitter. If from a learned transmitter, the message is decrypted and the synchro- nization counter is verified. Finally, the button status is checked to see what operation is requested. Figure 1-3 shows the relationship between some of the values stored by the receiver and the values received from the transmitter. FIGURE 1-2: BUILDING THE TRANSMITTED CODE WORD (ENCODER) Transmitter Manufacturer’s Serial Number Code Crypt Key Key Generation Algorithm Serial Number Crypt Key Sync Counter . . . HCS515 Production Programmer EEPROM Array Button Press Information EEPROM Array 32 Bits Encrypted Data Serial Number Transmitted Information Crypt Key Sync Counter Serial Number KEELOQ Encryption Algorithm |
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