HCS301 Datasheet, PDF (3/20 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER

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 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 1-2) 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 synchro-
nization counter. The synchronization value is then
combined with the encryption key in the encryption
algorithm and the output is 32 bits of encrypted infor-
mation. 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 4.3.
HCS301
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 HCS301. Section 7.0
provides more detail on integrating the HCS301 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.
FIGURE 1-2: BASIC OPERATION OF TRANSMITTER (ENCODER)
Transmitted Information
EEPROM Array
Encryption Key
Sync Counter
Serial Number
KEELOQ
Encryption
Algorithm
32 Bits of
Encrypted Data
Serial Number
Button Press
Information
FIGURE 1-3: BASIC OPERATION OF RECEIVER (DECODER)
EEPROM Array
Encryption Key
Sync Counter
Serial Number
Manufacturer Code
Check for
Match
KEELOQ
Encryption
Algorithm
Check for
Match
Decrypted
Synchronization
Counter
Button Press
Information
Serial Number
32 Bits of
Encrypted Data
Received Information
© 1996 Microchip Technology Inc.
Preliminary
DS21143A-page 3