HCS512-IP datasheet(3/20 Pages) MICROCHIP

3 / 20 page

HCS512

DS40151C-page 3

The 16-bit synchronization value is the basis for the

transmitted code changing for each transmission and is

updated each time a button is pressed. Because of the

complexity of the code hopping encryption algorithm, a

change in one bit of the synchronization value will result

in a large change in the actual transmitted code. There

is a relationship (Figure 1-3) between the key values in

EEPROM and how they are used in the encoder. Once

the encoder detects that a button has been pressed,

the encoder reads the button and updates the synchro-

nization counter. The synchronization value is then

combined with the encoder key in the encryption algo-

rithm, and the output is 32 bits of encrypted information.

This data will change with every button press, hence, it

is referred to as the hopping portion of the code word.

The 32-bit hopping code is combined with the button

information and the serial number to form the code

word transmitted to the receiver.

1.3

HCS Decoder Overview

Before a transmitter can be used with a particular

receiver, the transmitter must be ‘learned’ by the

receiver. Upon learning a transmitter, information is

stored by the receiver so that it may track the

transmitter,

including

the

serial

number

the

transmitter, the current synchronization value for that

transmitter, and the same encoder key that is used on

the transmitter. If a receiver receives a message of valid

format, the serial number is checked and, if it is from a

learned transmitter, the message is decrypted and the

decrypted synchronization counter is checked against

what is stored. If the synchronization value is veriﬁed,

then the button status is checked to see what operation

is needed. Figure 1-3 shows the relationship between

some of the values stored by the receiver and the val-

ues received from the transmitter.

FIGURE 1-2:

BASIC OPERATION OF TRANSMITTER (ENCODER)

FIGURE 1-3:

BASIC OPERATION OF RECEIVER (DECODER)

KEELOQ

Algorithm

Button Press

Information

Encryption

EEPROM Array

32 Bits of

Encrypted Data

Serial Number

Transmitted Information

Encoder Key

Sync Counter

Serial Number

Button Press

Information

EEPROM Array

Encoder Key

32-Bits of

Encrypted Data

Serial Number

Received Information

Decrypted

Synchronization

Counter

Check for

Match

Check for

Match

KEELOQ

Algorithm

Decryption

Sync Counter

Serial Number

Manufacturer’s Code

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