HCS515 datasheet(17/38 Pages) MICROCHIP

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DS40183E-page 17

HCS515

6.0

KEY GENERATION

The HCS515 supports two learning schemes which are selected during the initialization of the system EEPROM. The

learning schemes are:

• Normal learn using the KEELOQ decryption algorithm

• Secure learn using the KEELOQ decryption algorithm

6.1

Normal (Serial Number derived) Learn using the Decryption Algorithm

This learning scheme uses the KEELOQ decryption algorithm and the 28-bit serial number of the transmitter to derive

the encoder decryption key. The 28-bit serial number is patched with predefined values as indicated below to form two

32-bit seeds.

SourceH = 60000000 00000000H + Serial Number | 28 bits

SourceL = 20000000 00000000H + Serial Number | 28 bits

Then, using the KEELOQ decryption algorithm and the manufacturer’s code the encoder decryption key is

derived as follows:

KeyH Upper 32 bits = F KEELOQ Decryption (SourceH) | 64-bit Manufacturer’s Code

KeyL Lower 32 bits = F KEELOQ Decryption (SourceL) | 64-bit Manufacturer’s Code

6.2

Secure (Seed Derived) Learn using the Decryption Algorithm

This scheme uses the secure seed transmitted by the encoder to derive the two input seeds. The decoder always uses

the lower 64 bits of the transmission to form a 60-bit seed. The upper 4 bits are always forced to zero.

For 32-bit seed encoders:

SourceH = Serial Number Lower 28 bits

SourceL = Seed 32 bits

For 48-bit seed encoders:

SourceH = Seed Upper 16 bits + Serial Number Upper 16 bits (with upper 4 bits set to zero) << 16

SourceL = Seed Lower 32 bits

For 60-bit seed encoders:

SourceH = Seed Upper 28 bits with upper 4 bits set to zero

SourceL = Seed Lower 32 bits

The KEELOQ decryption algorithm and the manufacturer’s code is used to derive the encoder decryption key as

follows:

KeyH Upper 32 bits = Decrypt (SourceH) 64-bit Manufacturer’s Code

KeyL Lower 32 bits = Decrypt (SourceL) 64-bit Manufacturer’s Code

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