RFHCS362AF datasheet(37/60 Pages) MICROCHIP

37 / 60 page

Preliminary

DS41189A-page 37

rfHCS362G/362F

7.0

INTEGRATING THE rfHCS362G/

362F INTO THE SYSTEM

Use of the rfHCS362G/362F in a system requires a

compatible decoder. This decoder is typically a micro-

controller with compatible firmware. Microchip will pro-

vide (via a license agreement) firmware routines that

accept transmissions from the rfHCS362G/362F and

decrypt the hopping code portion of the data stream.

These routines provide system designers the means to

develop their own decoding system.

7.1

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 7-1 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 encryption 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

encryption key. The encoder receives its encryption

key during manufacturing. The decoder is programmed

with the ability to generate an encryption key as well as

all but one required input to the key generation routine;

typically the transmitter’s serial number.

Figure 7-1 summarizes a typical learn sequence. The

decoder receives and authenticates a first transmis-

sion; first button press. Authentication involves gener-

ating the appropriate encryption key, decrypting,

validating the correct key usage via the discrimination

bits and buffering the counter value. A second trans-

mission is received and authenticated. A final check

verifies the counter values were sequential; consecu-

tive button presses. If the learn sequence is success-

fully complete, the decoder stores the learned

transmitter’s serial number, current synchronization

counter value and appropriate encryption key. From

now on the encryption key will be retrieved from

EEPROM during normal operation instead of recalcu-

lating it for each transmission received.

Certain learning strategies have been patented and

care must be taken not to infringe.

FIGURE 7-1:

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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