RFHCS362F Datasheet, PDF (5/60 Pages) Microchip Technology – Code Hopping Encoder with UHF ASK/FSK Transmitter

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 an rfHCS362G/362F based transmitter.
Section 7.0 provides detail on integrating the
rfHCS362G/362F into a system.
rfHCS362G/362F
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
Production
Programmer
Transmitter
Serial Number
Manufacturer’s
Code
Key
Generation
Algorithm
Encryption
Key
rfHCS362
EEPROM Array
Serial Number
Encryption Key
Sync Counter
.
.
.
© 2002 Microchip Technology Inc.
Preliminary
DS41189A-page 5