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HCS512-P Datasheet(PDF) 3 Page - Microchip Technology |
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HCS512-P Datasheet(HTML) 3 Page - Microchip Technology |
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3 / 20 page  HCS512 © 1997 Microchip Technology Inc. DS40151C-page 3 The 16-bit synchronization value is the basis for the transmitted code changing for each transmission and is updated each time a button is pressed. Because of the complexity of the code hopping encryption algorithm, a change in one bit of the synchronization value will result in a large change in the actual transmitted code. There is a relationship (Figure 1-3) between the key values in EEPROM and how they are used in the encoder. Once the encoder detects that a button has been pressed, the encoder reads the button and updates the synchro- nization counter. The synchronization value is then combined with the encoder key in the encryption algo- rithm, and the output is 32 bits of encrypted information. This data will change with every button press, 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 the serial number to form the code word transmitted to the receiver. 1.3 HCS Decoder Overview Before a transmitter can be used with a particular receiver, the transmitter must be ‘learned’ by the receiver. Upon learning a transmitter, information is stored by the receiver so that it may track the transmitter, including the serial number of the transmitter, the current synchronization value for that transmitter, and the same encoder key that is used on the transmitter. If a receiver receives a message of valid format, the serial number is checked and, if it is from a learned transmitter, the message is decrypted and the decrypted synchronization counter is checked against what is stored. If the synchronization value is verified, then the button status is checked to see what operation is needed. Figure 1-3 shows the relationship between some of the values stored by the receiver and the val- ues received from the transmitter. FIGURE 1-2: BASIC OPERATION OF TRANSMITTER (ENCODER) FIGURE 1-3: BASIC OPERATION OF RECEIVER (DECODER) KEELOQ Algorithm Button Press Information Encryption EEPROM Array 32 Bits of Encrypted Data Serial Number Transmitted Information Encoder Key Sync Counter Serial Number Button Press Information EEPROM Array Encoder Key 32-Bits of Encrypted Data Serial Number Received Information Decrypted Synchronization Counter Check for Match Check for Match KEELOQ Algorithm Decryption Sync Counter Serial Number Manufacturer’s Code |
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