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HCS200-P Datasheet(PDF) 10 Page - Microchip Technology |
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HCS200-P Datasheet(HTML) 10 Page - Microchip Technology |
10 / 16 page HCS200 DS40138A-page 10 Preliminary © 1996 Microchip Technology Inc. 7.0 INTEGRATING THE HCS200 INTO A SYSTEM Use of the HCS200 in a system requires a compatible decoder. This decoder is typically a microcontroller with compatible firmware. Microchip will provide (via a license agreement) firmware routines that accept transmissions from the HCS200 and decrypt the hopping code portion of the data stream. These routines provide system designers the means to develop their own decoding system. 7.1 Learning a transmitter to a receiver In order for a transmitter to be used with a decoder, the transmitter must first be ‘learned’. Several learning strategies can be followed in the decoder implementa- tion. When a transmitter is learned to a decoder, it is suggested that the decoder stores the serial number and current synchronization value in EEPROM. The decoder must keep track of these values for every transmitter that is learned (Figure 7-1). The maximum number of transmitters that can be learned is only a function of how much EEPROM memory storage is available. The decoder must also store the manufac- turer’s code in order to learn a transmission transmitter, although this value will not change in a typical system so it is usually stored as part of the microcontroller ROM code. Storing the manufacturer’s code as part of the ROM code is also better for security reasons. It must be stated that some learning strategies have been patented and care must be taken not to infringe. FIGURE 7-1: TYPICAL LEARN SEQUENCE Enter Learn Mode Wait for Reception of a Valid Code Generate Key from Serial Number Use Generated Key to Decrypt Compare Discrimination Value with Fixed Value Equal Wait for Reception of Second Valid Code Compare Discrimination Value with Fixed Value Use Generated Key to Decrypt Equal Counters Encryption key Serial number Synchronization counter Sequential ? ? ? Exit Learn successful. Learn Unsuccessful No No No Yes Yes Yes Store: |
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