HCS500 datasheet(5/40 Pages) MICROCHIP

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DS40153D-page 5

HCS500

3.0

DECODER OPERATION

3.1

Learning a Transmitter to a

Receiver (Normal or Secure Learn)

Before the transmitter and receiver can work together,

the receiver must first ‘learn’ and store the following

information from the transmitter in EEPROM:

• A check value of the serial number

• The crypt key

• The current synchronization counter value

The decoder must also store the manufacturer’s code

(Section 1.1) in protected memory. This code will

typically be the same for all of the decoders in a sys-

tem.

The HCS500 has seven memory slots, and, conse-

quently, can store up to seven transmitters. During the

learn procedure, the decoder searches for an empty

memory slot for storing the transmitter’s information.

When all of the memory slots are full, the decoder will

overwrite the last transmitter’s information. To erase all

of the memory slots at once, use the ERASE_ALL com-

mand (C3H).

3.2

LEARNING PROCEDURE

Learning is initiated by sending the ACTIVATE_LEARN

(D2H) command to the decoder. The decoder acknowl-

edges reception of the command by pulling the data

line high.

For the HCS500 decoder to learn a new transmitter, the

following sequence is required:

Activate the transmitter once.

Activate the transmitter a second time. (In

Secure Learning mode, the seed transmission

must be transmitted during the second stage of

learn by activating the appropriate buttons on

the transmitter.)

The HCS500 will transmit a learn-status string,

indicating that the learn was successful.

The decoder has now learned the transmitter.

Repeat steps 1-3 to learn up to seven

transmitters

Note 1: Learning

will

terminated

two

nonsequential codes were received or if

two acceptable codes were not decoded

within 30 seconds.

2: If more than seven transmitters are

learned, the new transmitter will replace

the last transmitter learned. It is, therefore,

not possible to erase lost transmitters by

repeatedly learning new transmitters. To

remove

lost

stolen

transmitters,

ERASE_ALL transmitters and relearn all

available transmitters.

3: Learning a transmitter with a crypt key that

is identical to a transmitter already in mem-

ory replaces the existing transmitter. In

practice, this means that all transmitters

should have unique crypt keys. Learning a

previously learned transmitter does not use

any additional memory slots.

The following checks are performed by the decoder to

determine if the transmission is valid during learn:

• The first code word is checked for bit integrity.

• The second code word is checked for bit integrity.

• The crypt key is generated according to the

selected algorithm.

• The hopping code is decrypted.

• The discrimination value is checked.

• If all the checks pass, the key, serial number

check value, and synchronization counter values

are stored in EEPROM memory.

Figure 3-1 shows a flow chart of the learn sequence.

FIGURE 3-1:

LEARN SEQUENCE

Enter Learn

Mode

Wait for Reception

of Second

Compare Discrimination

Value with Serial Number

Use Generated Key

to Decrypt

Equal?

Sync. counter value

crypt key

Exit

Learn successful. Store:

Learn

Unsuccessful

Yes

Wait for Reception

of a Valid Code

Non-Repeated

Valid Code

Generate Key

from Serial Number/

Seed Value

Serial number check value

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