HCS410 datasheet(23/46 Pages) MICROCHIP

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HCS410

DS40158F-page 23

4.2

Learning an HCS410 to a Receiver

In order for a transmitter to be used with a decoder, the

transmitter must first be ‘learned’. Several learning

strategies can be followed in the decoder implementa-

tion. When a transmitter is learned to a decoder, it is

suggested that the decoder stores the serial number

and current synchronization counter value (synchroni-

zation counter stored in CH Mode only) in EEPROM.

The decoder must keep track of these values for every

transmitter that is learned (Figure 4-2 and Figure 4-3).

FIGURE 4-2:

TYPICAL CH MODE LEARN

SEQUENCE

The maximum number of transmitters that can be

learned is only a function of how much EEPROM

memory storage is available. The decoder must also

store the manufacturer’s code in order to learn an

HCS410, although this value will not change in a typical

system so it is usually stored as part of the microcon-

troller ROM code. Storing the manufacturer’s code as

part of the ROM code is also better for security rea-

sons.

FIGURE 4-3:

TYPICAL IFF 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

Encoder key

Serial number

Synchronization counter

Sequential

Exit

Learn successful Store:

Learn

Unsuccessful

Yes

Enter Learn

Wait for token

to be detected

Read

Generate Key

From Serial

Perform IFF

with Token

Compare Token

and expected

response

Token and

Response

Equal?

Exit

Serial Number

Yes

Learn successful

Serial number

Encoder key

Number

Store:

Mode

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