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HCS301-IP Datasheet(PDF) 5 Page - Microchip Technology |
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HCS301-IP Datasheet(HTML) 5 Page - Microchip Technology |
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5 / 20 page  © 1996 Microchip Technology Inc. Preliminary DS21143A-page 5 HCS301 3.0 EEPROM MEMORY ORGANIZATION The HCS301 contains 192 bits (12 x 16-bit words) of EEPROM memory (Table 3-1). This EEPROM array is used to store the encryption key information, synchronization value, etc. Further descriptions of the memory array is given in the following sections. TABLE 3-1: EEPROM MEMORY MAP 3.1 Key_0 - Key_3 (64-Bit Encryption Key) The 64-bit encryption key is used by the transmitter to create the encrypted message transmitted to the receiver. This key is created and programmed at the time of production using a key generation algorithm. The key generation algorithm is different from the KEELOQ algorithm, although it too is a proprietary encryption method. Inputs to the key generation algorithm are the serial number for the particular transmitter being used and the 64-bit manufacturer’s code. While the key generation algorithm supplied from Microchip is the typical method used, a user may elect to create their own method of key generation. This may be done providing that the decoder is programmed with the same means of creating the key for decryption purposes. WORD ADDRESS MNEMONIC DESCRIPTION 0 KEY_0 64-bit encryption key (word 0) 1 KEY_1 64-bit encryption key (word 1) 2 KEY_2 64-bit encryption key (word 2) 3 KEY_3 64-bit encryption key (word 3) 4 SYNC 16-bit synchronization value 5 RESERVED Set to 0000H 6 SER_0 Device Serial Number (word 0) 7 SER_1(Note) Device Serial Number (word 1) 8 SEED_0 Seed Value (word 0) 9 SEED_1 Seed Value (word 1) 10 EN_KEY 16-bit Envelope Key 11 CONFIG Configuration Word Note: The MSB of the serial number contains a bit used to select the auto shutoff timer. 3.2 SYNC (Synchronization Counter) This is the 16-bit synchronization value that is used to create the hopping code for transmission. This value will be changed after every transmission. 3.3 SER_0, SER_1 (Encoder Serial Number) SER_0 and SER_1 are the lower and upper words of the device serial number, respectively. Although there are 32 bits allocated for the serial number, only the lower order 28 bits are transmitted. The serial number is meant to be unique for every transmitter. The most significant bit of the serial number (Bit 31) is used to turn the auto shutoff timer on or off. 3.3.1 AUTO-SHUTOFF TIMER SELECT The most significant bit of the serial number (Bit 31) is used to turn the Auto-shutoff timer on or off. This timer prevents the transmitter from draining the battery should a button get stuck in the on position for a long period of time. The time period is approximately 25 seconds, after which the device will go to the Time-out mode. When in the Time-out mode, the device will stop transmitting, although since some circuits within the device are still active, the current draw within the Shutoff mode will be more than Standby mode. If the most significant bit in the serial number is a one, then the Auto-shutoff timer is enabled, and a zero in the most significant bit will disable the timer. The length of the timer is not selectable. 3.4 SEED_0, SEED_1 (Seed Word) This is the two-word (32 bits) seed code that will be transmitted when all four buttons are pressed at the same time. This allows the system designer to implement the secure learn feature or use this fixed code word as part of a different key generation/tracking process. 3.5 EN_Key (Envelope Encryption Key) Envelope encryption is a selectable option that encrypts the portion of the transmission that contains the transmitter serial number and function code. Select- ing this option is done by setting the appropriate bit in the configuration word (Table 3-2). Normally, the serial number and function code are transmitted in the clear (unencrypted), but for an added level of security, the system designer may elect to implement this option. The envelope encryption key is used to encrypt the serial number and function code portion of the trans- mission, if the envelope encryption option has been selected. The envelope encryption algorithm is a differ- ent algorithm than the key generation or transmit encryption algorithm. The EN_key is typically a random number and the same for all transmitters in a system. |
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