HCS512-ISN MICROCHIP [Microchip Technology], HCS512-ISN Datasheet - Page 3

• Manufacturer Part Number: HCS512-ISN
• Description: KEELOQ CODE HOPPING DECODER
• Manufacturer: MICROCHIP [Microchip Technology]

The 16-bit synchronization value is the basis for the
transmitted code changing for each transmission and is
updated each time a button is pressed. Because of the
complexity of the code hopping encryption algorithm, a
change in one bit of the synchronization value will result
in a large change in the actual transmitted code. There
is a relationship (Figure 1-3) between the key values in
EEPROM and how they are used in the encoder. Once
the encoder detects that a button has been pressed,
the encoder reads the button and updates the synchro-
nization counter. The synchronization value is then
combined with the encoder key in the encryption algo-
rithm, and the output is 32 bits of encrypted information.
This data will change with every button press, hence, it
is referred to as the hopping portion of the code word.
The 32-bit hopping code is combined with the button
information and the serial number to form the code
word transmitted to the receiver.
FIGURE 1-2:
FIGURE 1-3:
1997 Microchip Technology Inc.
EEPROM Array
Serial Number
Sync Counter
Encoder Key
Manufacturer’s Code
EEPROM Array
BASIC OPERATION OF TRANSMITTER (ENCODER)
BASIC OPERATION OF RECEIVER (DECODER)
Serial Number
Sync Counter
Encoder Key
Button Press
Information
Encryption
Algorithm
K
EE
Received Information
L
OQ
Serial Number
Check for
Match
Encrypted Data
32 Bits of
1.3
Before a transmitter can be used with a particular
receiver, the transmitter must be ‘learned’ by the
receiver. Upon learning a transmitter, information is
stored by the receiver so that it may track the
transmitter, including the serial number of the
transmitter, the current synchronization value for that
transmitter, and the same encoder key that is used on
the transmitter. If a receiver receives a message of valid
format, the serial number is checked and, if it is from a
learned transmitter, the message is decrypted and the
decrypted synchronization counter is checked against
what is stored. If the synchronization value is verified,
then the button status is checked to see what operation
is needed. Figure 1-3 shows the relationship between
some of the values stored by the receiver and the val-
ues received from the transmitter.
Encrypted Data
Decryption
Algorithm
32-Bits of
K
EE
L
HCS Decoder Overview
OQ
Transmitted Information
Serial Number
Synchronization
Check for
Decrypted
Match
Counter
HCS512
Button Press
Information
DS40151C-page 3