© 1996 Microchip Technology Inc.

Preliminary

DS40138A-page 3

HCS200

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 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 1-2) 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 encryption key in the encryption

algorithm and the output is 32 bits of encrypted infor-

mation. 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. The code word

format is explained in detail in Section 4.3.

Any type of controller may be used as a receiver, but it

is typically a microcontroller with compatible firmware

that allows the receiver to operate in conjunction with a

transmitter based on the HCS200. Section 7.0 provides

more detail on integrating the HCS200 into a total sys-

tem.

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

of

the

transmitter, the current synchronization value for that

transmitter, and the same encryption 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 values received from the transmitter.

FIGURE 1-2:

BASIC OPERATION OF TRANSMITTER (ENCODER)

FIGURE 1-3:

BASIC OPERATION OF RECEIVER (DECODER)

Button Press

Information

EEPROM Array

32 Bits of

Encrypted Data

Serial Number

Transmitted Information

Encryption Key

Sync Counter

Serial Number

KeeLoq

Encryption

Algorithm

Button Press

Information

EEPROM Array

Encryption Key

32 Bits of

Encrypted Data

Serial Number

Received Information

Decrypted

Synchronization

Counter

Check for

Match

Check for

Match

Sync Counter

Serial Number

Manufacturer Code

Decryption

Algorithm