HCS362 datasheet(22/46 Pages) MICROCHIP

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HCS362

DS40189E-page 22

6.0

INTEGRATING THE HCS362

INTO A SYSTEM

Use of the HCS362 in a system requires a compatible

decoder. This decoder is typically a microcontroller with

compatible firmware. Microchip will provide (via a

license agreement) firmware routines that accept

transmissions from the HCS362 and decrypt the

hopping code portion of the data stream. These

routines provide system designers the means to

develop their own decoding system.

6.1

Learning a Transmitter to a

Receiver

A transmitter must first be 'learned' by a decoder before

its use is allowed in the system. Several learning strat-

egies are possible, Figure 6-1 details a typical learn

sequence. Core to each, the decoder must minimally

store each learned transmitter's serial number and cur-

rent synchronization counter value in EEPROM. Addi-

tionally, the decoder typically stores each transmitter's

unique crypt key. The maximum number of learned

transmitters will therefore be relative to the available

EEPROM.

A transmitter's serial number is transmitted in the clear

but the synchronization counter only exists in the code

word's encrypted portion. The decoder obtains the

counter value by decrypting using the same key used

to encrypt the information. The KEELOQ algorithm is a

symmetrical block cipher so the encryption and decryp-

tion keys are identical and referred to generally as the

crypt key. The encoder receives its crypt key during

manufacturing. The decoder is programmed with the

ability to generate a crypt key as well as all but one

required input to the key generation routine; typically

the transmitter's serial number.

Figure 6-1 summarizes a typical learn sequence. The

decoder receives and authenticates a first transmis-

sion; first button press. Authentication involves gener-

ating the appropriate crypt key, decrypting, validating

the correct key usage via the discrimination bits and

buffering the counter value. A second transmission is

received and authenticated. A final check verifies the

counter values were sequential; consecutive button

presses. If the learn sequence is successfully com-

plete, the decoder stores the learned transmitter's

serial number, current synchronization counter value

and appropriate crypt key. From now on the crypt key

will be retrieved from EEPROM during normal opera-

tion instead of recalculating it for each transmission

received.

Certain learning strategies have been patented and

care must be taken not to infringe.

FIGURE 6-1:

TYPICAL LEARN

SEQUENCE

Enter Learn

Mode

Wait for Reception

of a Valid Code

Generate Key

from Serial Number

Use Generated Key

to Decrypt

Compare Discrimination

Value with Fixed Value

Equal

Wait for Reception

of Second Valid Code

Compare Discrimination

Value with Fixed Value

Use Generated Key

to Decrypt

Equal

Counters

Encryption key

Serial number

Synchronization counter

Sequential

Exit

Learn successful Store:

Learn

Unsuccessful

Yes

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