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HCS365 Datasheet(PDF) 3 Page - Microchip Technology |
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HCS365 Datasheet(HTML) 3 Page - Microchip Technology |
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3 / 34 page  © 2002 Microchip Technology Inc. Preliminary DS41109D-page 3 HCS365 As indicated in the block diagram on page one, the HCS365 has a small EEPROM array which must be loaded with several parameters before use; most often programmed by the manufacturer at the time of produc- tion. The most important of these are: • A serial number, typically unique for every encoder • A crypt key • An initial synchronization value The crypt key generation typically inputs the transmitter serial number and 64-bit manufacturer’s code into the key generation algorithm (Figure 1-1). The manufac- turer’s code is chosen by the system manufacturer and must be carefully controlled as it is a pivotal part of the overall system security. FIGURE 1-1: CREATION AND STORAGE OF CRYPT KEY DURING PRODUCTION The valid synchronization counter is the basis behind the transmitted code word changing for each transmis- sion; it increments each time a button is pressed. Each increment of the synchronization value results in more than 50% of the hopping code bits changing. 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 while its value will appear to ‘randomly hop around’. Hence, this data 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 4.1. A receiver may use any type of controller as a decoder. Typically, it is a microcontroller with compatible firm- ware that allows the decoder to operate in conjunction with an HCS365 based transmitter. 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, storing the serial number, storing the synchronization counter value, and storing 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 the serial num- ber is from a learned transmitter, the message is decrypted and the synchronization counter is verified. Finally, the button status is checked to see what opera- tion is requested. Figure 1-3 shows the relationship between some of the values stored by the receiver and the values received from the transmitter. For detailed decoder operation, see Section 7.0. Transmitter Manufacturer’s Serial Number Code Crypt Key Key Generation Algorithm Serial Number Crypt Key Sync Counter . . . HCS365 Production Programmer EEPROM Array |
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