HCS500T-ISLASHST datasheet(3/42 Pages) MICROCHIP

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DS40000153E-page 3

HCS500

FIGURE 1-1:

CREATION AND STORAGE OF CRYPT KEY DURING PRODUCTION

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 “Code Word Organiza-

tion”.

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 HCS500 based transmitter. Section 3.0

“Decoder Operation” provides detail on integrating

the HCS500 into a system.

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)

Transmitter

Manufacturer’s

Serial Number

Code

Crypt

Key

Generation

Algorithm

Serial Number

Crypt Key

Sync Counter

HCS500

Production

Programmer

EEPROM Array

Button Press

Information

EEPROM Array

32 Bits

Encrypted Data

Serial Number

Transmitted Information

Crypt Key

Sync Counter

Serial Number

KEELOQ

Encryption

Algorithm

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