HCS362 datasheet(3/46 Pages) MICROCHIP

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DS40189E-page 3

HCS362

the HCS362 is based on the patented KEELOQ technol-

ogy. A block cipher based on a block length of 32 bits

and a key length of 64 bits is used. The algorithm

obscures the information in such a way that even if the

transmission information (before coding) differs by only

one bit from that of the previous transmission, the next

coded transmission will be completely different. Statis-

tically, if only one bit in the 32-bit string of information

changes, greater than 50 percent of the coded trans-

mission bits will change.

As indicated in the block diagram on page one, the

HCS362 has a small EEPROM array which must be

loaded with several parameters before use; most often

programmed by the manufacturer at the time of produc-

tion. The most important of these are:

• A 28-bit serial number, typically unique for every

encoder

• A crypt key

• An initial 16-bit synchronization value

• A 16-bit configuration value

The crypt key generation typically inputs the transmitter

serial number and 64-bit manufacturer’s code into the

key generation algorithm (Figure 1-1). The manufac-

turer’s code is chosen by the system manufacturer and

must be carefully controlled as it is a pivotal part of the

overall system security.

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 3.1.

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 HCS362 based transmitter. Section 6.0

provides detail on integrating the HCS362 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.

Transmitter

Manufacturer’s

Serial Number

Code

Crypt

Key

Generation

Algorithm

Serial Number

Crypt Key

Sync Counter

HCS362

Production

Programmer

EEPROM Array

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