HCS300-IP Microchip Technology, HCS300-IP Datasheet - Page 3

• Manufacturer Part Number: HCS300-IP
• Description: KEE LOQ Code Hopping Encoder
• Manufacturer: 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 algo-
rithm, 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 1-2) 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 synchronization counter. The synchroniza-
tion value is then combined with the encryption key in
the encryption algorithm 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. The code word format is explained in detail
in Section 4.2.
FIGURE 1-2:
FIGURE 1-3:
1996 Microchip Technology Inc.
EEPROM Array
Encryption Key
Serial Number
Sync Counter
Manufacturer Code
EEPROM Array
BASIC OPERATION OF TRANSMITTER (ENCODER)
BASIC OPERATION OF RECEIVER (DECODER)
Encryption Key
Serial Number
Sync Counter
Button Press
Information
Encryption
Algorithm
K
EE
L
Received Information
OQ
Serial Number
Check for
Match
Preliminary
Encrypted Data
32 Bits of
Any type of controller may be used as a receiver, but it
is typically a microcontroller with compatible firmware
that allows the receiver to operate in conjunction with a
transmitter, based on the HCS300. Section 7.0
provides more detail on integrating the HCS300 into a
total system.
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 encryption 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
Transmitted Information
L
OQ
Serial Number
Synchronization
Check for
Decrypted
Match
Counter
Button Press
Information
HCS300
DS21137D-page 3