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HCS512-P Datasheet(PDF) 6 Page - Microchip Technology |
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HCS512-P Datasheet(HTML) 6 Page - Microchip Technology |
6 / 20 page HCS512 DS40151C-page 6 © 1997 Microchip Technology Inc. 4.0 DECODER OPERATION 4.1 Learning a Transmitter to a Receiver Either the serial number-based learning method or the seed-based learning method can be selected. The learning method is selected in the configuration byte. In order for a transmitter to be used with a decoder, the transmitter must first be ‘learned’. When a transmitter is learned to a decoder, the decoder stores the encoder key, a check value of the serial number and current syn- chronization value in EEPROM. The decoder must keep track of these values for every transmitter that is learned. The maximum number of transmitters that can be learned is four. The decoder must also contain the Manufacturer’s Code in order to learn a transmitter. The Manufacturer’s Code will typically be the same for all decoders in a system. The HCS512 has four memory slots. After an “erase all” procedure, all the memory slots will be cleared. Erase all is activated by taking LRNIN low for approximately 10 seconds. When a new transmitter is learned, the decoder searches for an empty memory slot and stores the transmitter’s information in that memory slot. When all memory slots are full, the decoder randomly over- writes existing transmitters. 4.1.1 LEARNING PROCEDURE Learning is activated by taking the LRNIN input low for longer than 64 ms. This input requires an external pull- up resistor. To learn a new transmitter to the HCS512 decoder, the following sequence is required: 1. Enter learning mode by pulling LRNIN low for longer than 64 ms. The LRNOUT output will go high. 2. Activate the transmitter until the LRNOUT output goes low indicating reception of a valid code (hopping message). 3. Activate the transmitter a second time until the LRNOUT toggles for 4 seconds (in secure learn- ing mode, the seed transmission must be trans- mitted during the second stage of learn by activating the appropriate buttons on the trans- mitter). If LRNIN is taken low momentarily during the learn status indication, the indication will be ter- minated. Once a successful learning sequence is detected, the indication can be terminated allowing quick learning in a manufacturing setup. 4. The transmitter is now learned into the decoder. 5. Repeat steps 1-4 to learn up to four transmitters. 6. Learning will be terminated if two non-sequential codes were received or if two acceptable codes were not decoded within 30 seconds. The following checks are performed on the decoder to determine if the transmission is valid during learn: • The first code word is checked for bit integrity. • The second code word is checked for bit integrity. • The hopping code is decrypted. • If all the checks pass, the serial number and syn- chronization counters are stored in EEPROM memory. Figure 4-1 shows a flow chart of the learn sequence. FIGURE 4-1: LEARN SEQUENCE 4.2 Validation of Codes The decoder waits for a transmission and checks the serial number to determine if the transmitter has been learned. If learned, the decoder decrypts the encrypted portion of the transmission using the encoder key. It uses the discrimination bits to determine if the decryp- tion was valid. If everything up to this point is valid, the synchronization value is evaluated. Enter Learn Mode Wait for Reception of Second Compare Discrimination Value with Serial Number Use Generated Key to Decrypt Equal Serial number check value Synchronization counter ? Exit Learn successful. Store: Learn Unsuccessful No Yes Wait for Reception of a Valid Code Non-Repeated Valid Code Generate Key from Serial Number or Seed Value Encoder Key |
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