HCS512_11 datasheet(13/34 Pages) MICROCHIP

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DS40151E-page 13

HCS512

7.3

Key Generation Algorithms

There are two key generation algorithms implemented in the HCS512 decoder. The KEELOQ decryption algorithm pro-

vides a higher level of security than the XOR algorithm. Section 6.1 describes the selection of the algorithms in the con-

figuration byte.

7.3.1

KEELOQ DECRYPT ALGORITHM

This algorithm uses the KEELOQ decryption algorithm and the manufacturer’s code to derive the crypt key as follows:

Key Upper 32 bits = Decrypt (SourceH) 64 Bit Manufacturers Code

Key Lower 32 bits = Decrypt (SourceL) 64 Bit Manufacturers Code

7.3.2

XOR WITH THE MANUFACTURER’S CODE

The two 32-bits seeds are XOR with the manufacturer’s code to form the 64 bit crypt key.

Key Upper 32 bits = SourceH XOR Manufacturers Code Upper 32 bits

Key Lower 32 bits = SourceL XOR Manufacturers Code Lower 32 bits

After programming the manufacturer’s code, the HCS512 decoder will automatically activate an Erase All function,

removing all transmitters from the system.

If LRNIN is taken low momentarily during the learn status indication, the indication will be terminated. Once a successful

learning sequence is detected, the indication can be terminated, allowing quick learning in a manufacturing setup.

FIGURE 7-2:

HCS512

KEY GENERATION

KEELOQ®

Decryption

Algorithm

Padding

28-bit Serial Number

Padding

28-bit Serial Number

MS 32 bits of crypt key

LS 32 bits of crypt key

Normal Learn (SC_LRN = 0)

LRN0 = 0

KEELOQ

Decryption

Algorithm

Padding

0000b

MS 28 bits of Seed Transmission

LS 32 bits of Seed Transmission

MS 32 bits of crypt key

LS 32 bits of crypt key

Secure Learn (SC_LRN = 1)

XOR

Padding

0000b

MS 28 bits of Seed Transmission

LS 32 bits of Seed Transmission

MS 32 bits of crypt key

LS 32 bits of crypt key

LRN0 = 1

Secure Learn XOR (SC_LRN = 1)

LRN0 = 0

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