HCS365 datasheet(3/34 Pages) MICROCHIP

3 / 34 page

Preliminary

DS41109D-page 3

HCS365

As indicated in the block diagram on page one, the

HCS365 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 serial number, typically unique for every

encoder

• A crypt key

• An initial synchronization 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 valid synchronization counter is the basis behind

the transmitted code word changing for each transmis-

sion; it increments each time a button is pressed. Each

increment of the synchronization value results in more

than 50% of the hopping code bits changing.

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 while its value will

appear to ‘randomly hop around’. Hence, this data 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 4.1.

A receiver may use any type of controller as a decoder.

Typically, it is a microcontroller with compatible firm-

ware that allows the decoder to operate in conjunction

with an HCS365 based transmitter.

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, storing the

serial number, storing the synchronization counter

value, and storing 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 the serial num-

ber is from a learned transmitter, the message is

decrypted and the synchronization counter is verified.

Finally, the button status is checked to see what opera-

tion is requested. Figure 1-3 shows the relationship

between some of the values stored by the receiver and

the values received from the transmitter.

For detailed decoder operation, see Section 7.0.

Transmitter

Manufacturer’s

Serial Number

Code

Crypt

Key

Generation

Algorithm

Serial Number

Crypt Key

Sync Counter

HCS365

Production

Programmer

EEPROM Array

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