HCS500-ISM datasheet(3/24 Pages) MICROCHIP

3 / 24 page

HCS500

Preliminary

DS40153B-page 3

The 16-bit synchronization counter 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 KEELOQ encryption algorithm, a

change in one bit of the synchronization counter value

will result in a large change in the actual transmitted

code. There is a relationship (Figure 1-2) between the

encoder 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 synchronization counter. The synchroniza-

tion value is then combined with the encoder key in the

KEELOQ encryption algorithm, and the output is 32 bits

of encrypted information. This data will change with

every button press, hence, it is referred to as the code

hopping portion of the code word. The 32-bit code hop-

ping portion is combined with the button information

and the serial number to form the code word transmit-

ted to the receiver.

1.3

HCS Decoder Overview

Before a transmitter and receiver can work together, the

receiver must ﬁrst ‘learn’ and store certain information

from the transmitter. This information includes a ‘check

value’ of the serial number, the encoder key, and cur-

rent synchronization counter value.

When a validly formatted message is detected, the

receiver ﬁrst compares the serial number. If the serial

number check value is from a learned transmitter, the

message is decrypted. Next, the receiver checks the

decrypted synchronization counter value against what

is stored in memory. If the synchronization counter

value is veriﬁed, then a valid transmission message is

sent. Figure 1-3 shows the relationship between some

of the values stored by the receiver and the values

received from the transmitter.

FIGURE 1-2:

BASIC OPERATION OF A CODE HOPPING TRANSMITTER (ENCODER)

FIGURE 1-3:

BASIC OPERATION OF A CODE HOPPING RECEIVER (DECODER)

KEELOQ

Algorithm

Button Press

Information

Encryption

EEPROM Array

32 Bits of

Encrypted Data

Serial Number

Transmitted Information

Encoder Key

Sync. Counter Value

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 Value

Serial Number

Manufacturer Code

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