© 2002 Microchip Technology Inc.

DS40151D-page 13

HCS512

7.3

Key Generation Algorithms

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

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.

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

Decryption

Algorithm

Padding

6

28-bit Serial Number

Padding

2

28-bit Serial Number

MS 32 bits of crypt key

LS 32 bits of crypt key

Normal Learn (SC_LRN = 0)

LRN0 = 0

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