HCS515 View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

HCS515

KEELOQ Code Hopping Decoder

Microchip Technology 

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 encoder decryption key

• The current synchronization counter value

The decoder must also store the manufacturer’s code

(Section 1.2) in protected memory. This code will

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

tem.

The HCS515 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

command (C3H).

3.1.1 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 HCS515 decoder to learn a new transmitter, the

following sequence is required:

1. Activate the transmitter once.

2. 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 HCS515 will transmit a learn-status string,

indicating that the learn was successful.

3. The decoder has now learned the transmitter.

4. Repeat steps 1-3 to learn up to seven

transmitters

Note 1: Learning will be terminated if 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 pos-

sible to erase lost transmitters by

repeatedly learning new transmitters. To

remove lost or stolen transmitters,

ERASE_ALL transmitters and relearn all

available transmitters.

HCS515

3: Learning a transmitter with an encoder

decryption key that is identical to a transmit-

ter already in memory replaces the existing

transmitter. In practice, this means that all

transmitters should have unique encoder

decryption keys. Learning a previously

learned transmitter does not use any addi-

tional 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 encoder decryption key is generated accord-

ing 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 a Valid Code

Wait for Reception

of Second

Non-Repeated

Valid Code

Generate Key

from Serial Number/

Seed Value

Use Generated Key

to Decrypt

Compare Discrimination

Value with Serial Number

Equal? No

Yes

Learn successful. Store:

Serial number check value

Encoder decryption key

Sync. counter value

Learn

Unsuccessful

Exit

© 1998 Microchip Technology Inc.

Preliminary

DS40183A-page 5

Share Link: