HCS512 Datasheet, PDF (12/20 Pages) Microchip Technology – KEELOQ CODE HOPPING DECODER

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 encoder key as follows:
Key _{Upper 32 bits} = F _{KEELOQ Decrypt} (SourceH) | _{64 Bit Manufacturers Code}
Key _{Lower 32 bits} = F _{KEELOQ 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 encoder key.
Key _{Upper 32 bits} = SourceH ⊗ Manufacturers Code | _{Upper 32 bits}
Key _{Lower 32 bits} = SourceL ⊗ 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 set up.
FIGURE 7-2: HCS512 KEY GENERATION
Padding
2/2B
SC_LRN = 0
28/24-bit Serial Number
Padding
6/65
28/24-bit Serial Number
LRN0 = 0
KEELOQ
Decryption
Algorithm
LS 32 bits of Encoder Key
MS 32 bits of Encoder Key
SC_LRN = 1
LS 32 bits of Seed Transmission
Padding
0000b
MS 28 bits of Seed Transmission
KEELOQ
Decryption
Algorithm
LS 32 bits of Encoder Key
MS 32 bits of Encoder Key
LS 32 bits of Seed Transmission
Padding
0000b
MS 28 bits of Seed Transmission
LRN0 = 1
XOR
LS 32 bits of Encoder Key
MS 32 bits of Encoder Key
DS40151C-page 12
© 1997 Microchip Technology Inc.