RFHCS362G datasheet(4/54 Pages) MICROCHIP

4 / 54 page

rfHCS362G/362F

DS41189B-page 4

Figure 1-3 shows how the key values in EEPROM are

used in the encoder. Once the encoder detects a button

press, it reads the button inputs and updates the syn-

chronization counter. The synchronization counter and

encryption key are input to the encryption algorithm

and the output is 32 bits of encrypted information. This

data will change with every button press, its value

appearing externally to ‘randomly hop around’, 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 serial number to form the code word

transmitted to the receiver. The code word format is

explained in greater detail in Section 3.1.

A receiver may use any type of controller as a decoder,

but it is typically a microcontroller with compatible firm-

ware that allows the decoder to operate in conjunction

with

rfHCS362G/362F

based

transmitter.

Section 7.0

provides

detail

integrating

the

rfHCS362G/362F into a system.

A transmitter must first be ‘learned’ by the receiver

before its use is allowed in the system. Learning

includes calculating the transmitter’s appropriate

encryption key, decrypting the received hopping code

and storing the serial number, synchronization counter

value and encryption key in EEPROM.

In normal operation, each received message of valid

format is evaluated. The serial number is used to deter-

mine if it is from a learned transmitter. If from a learned

transmitter, the message is decrypted and the synchro-

nization counter is verified. Finally, the button status is

checked to see what operation is requested. Figure 1-4

shows the relationship between some of the values

stored by the receiver and the values received from

the transmitter.

FIGURE 1-2:

CREATION AND STORAGE OF ENCRYPTION KEY DURING PRODUCTION

Transmitter

Manufacturer’s

Serial Number

Code

Encryption

Key

Generation

Algorithm

Serial Number

Encryption Key

Sync Counter

rfHCS362

Production

Programmer

EEPROM Array

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