HCS515_11 Datasheet, PDF (3/38 Pages) Microchip Technology – KEELOQ® Code Hopping Decoder stand-alone decoder

HCS515
FIGURE 1-1:
CREATION AND STORAGE OF CRYPT KEY DURING PRODUCTION
Production
Programmer
Transmitter
Serial Number
Manufacturer’s
Code
Key
Generation
Algorithm
Crypt
Key
HCS515
EEPROM Array
Serial Number
Crypt Key
Sync Counter
.
.
.
The 16-bit synchronization counter is the basis behind
the transmitted code word changing for each transmis-
sion; it increments each time a button is pressed. Due
to the code hopping algorithm’s complexity, each incre-
ment of the synchronization value results in greater
than 50% of the bits changing in the transmitted code
word.
Figure 1-2 shows how the key values in EEPROM are
used in the encoder. Once the encoder detects a button
press, it reads the button inputs and updates the syn-
chronization counter. The synchronization counter and
crypt key are input to the encryption algorithm and the
output is 32 bits of encrypted information. This data will
change with every button press, its value appearing
externally to ‘randomly hop around’, 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 serial number to form the code word transmitted to
the receiver. The code word format is explained in
greater detail in Section 7.2.
A receiver may use any type of controller as a decoder,
but it is typically a microcontroller with compatible firm-
ware that allows the decoder to operate in conjunction
with an HCS515 based transmitter. Section 3.0
provides detail on integrating the HCS515 into a sys-
tem.
A transmitter must first be ‘learned’ by the receiver
before its use is allowed in the system. Learning
includes calculating the transmitter’s appropriate crypt
key, decrypting the received hopping code and storing
the serial number, synchronization counter value and
crypt key in EEPROM.
In normal operation, each received message of valid
format is evaluated. The serial number is used to deter-
mine if it is from a learned transmitter. If from a learned
transmitter, the message is decrypted and the synchro-
nization counter is verified. Finally, the button status is
checked to see what operation is requested. 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:
BUILDING THE TRANSMITTED CODE WORD (ENCODER)
EEPROM Array
Crypt Key
Sync Counter
Serial Number
KEELOQ®
Encryption
Algorithm
Button Press
Information
Serial Number
32 Bits
Encrypted Data
Transmitted Information
© 2011 Microchip Technology Inc.
DS40183E-page 3