HCS370SL datasheet(18/36 Pages) MICROCHIP

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HCS370

DS41111D-page 18

Preliminary

2002 Microchip Technology Inc.

7.0

INTEGRATING THE HCS370

INTO A SYSTEM

Use of the HCS370 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 HCS370 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. The decoder must minimally store each

learned transmitter’s serial number and current syn-

chronization counter value in EEPROM. Additionally,

the decoder typically stores each transmitter’s unique

crypt key. The maximum number of learned transmit-

ters will therefore be relative to the available EEPROM.

A transmitter’s serial number is transmitted in the 32-bit

fixed code, 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 algo-

rithm is a symmetrical block cipher so the encryption

and decryption keys are identical and referred to gen-

erally as the crypt key. The encoder receives its crypt

key during manufacturing. The decoder typically calcu-

lates the crypt key by running the encoder serial num-

ber or seed through the key generation routine.

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 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-

pleted, 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 by 3rd

parties 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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