HCS500 Datasheet, PDF (3/24 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER

HCS500
The 16-bit synchronization counter 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 KEELOQ encryption algorithm, a
change in one bit of the synchronization counter value
will result in a large change in the actual transmitted
code. There is a relationship (Figure 1-2) between the
encoder 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 encoder key in the
KEELOQ 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 code
hopping portion of the code word. The 32-bit code hop-
ping portion is combined with the button information
and the serial number to form the code word transmit-
ted to the receiver.
1.3 HCS Decoder Overview
Before a transmitter and receiver can work together, the
receiver must first ‘learn’ and store certain information
from the transmitter. This information includes a ‘check
value’ of the serial number, the encoder key, and cur-
rent synchronization counter value.
When a validly formatted message is detected, the
receiver first compares the serial number. If the serial
number check value is from a learned transmitter, the
message is decrypted. Next, the receiver checks the
decrypted synchronization counter value against what
is stored in memory. If the synchronization counter
value is verified, then a valid transmission message is
sent. Figure 1-3 shows the relationship between some
of the values stored by the receiver and the values
received from the transmitter.
FIGURE 1-2: BASIC OPERATION OF A CODE HOPPING TRANSMITTER (ENCODER)
Transmitted Information
EEPROM Array
Encoder Key
Sync. Counter Value
Serial Number
KEELOQ
Encryption
Algorithm
32 Bits of
Encrypted Data
Serial Number
Button Press
Information
FIGURE 1-3: BASIC OPERATION OF A CODE HOPPING RECEIVER (DECODER)
EEPROM Array
Encoder Key
Sync. Counter Value
Serial Number
Manufacturer Code
Check for
Match
KEELOQ
Decryption
Algorithm
Check for
Match
Decrypted
Synchronization
Counter
Button Press
Information
Serial Number
32 Bits of
Encrypted Data
Received Information
© 1997 Microchip Technology Inc.
Preliminary
DS40153B-page 3