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HCS370IST Datasheet(PDF) 3 Page - Microchip Technology

Part # HCS370IST
Description  KEELOQ Code Hopping Encoder
Download  36 Pages
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Manufacturer  MICROCHIP [Microchip Technology]
Direct Link  http://www.microchip.com
Logo MICROCHIP - Microchip Technology

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2002 Microchip Technology Inc.
Preliminary
DS41111D-page 3
HCS370
As indicated in the block diagram on page one, the
HCS370 has a small EEPROM array which must be
loaded with several parameters before use; most often
programmed by the manufacturer at the time of produc-
tion. The most important of these are:
• A serial number, typically unique for every
encoder
• A crypt key
• An initial synchronization value
The crypt key generation typically inputs the transmitter
serial number and 64-bit manufacturer’s code into the
key generation algorithm (Figure 1-1). The manufac-
turer’s code is chosen by the system manufacturer and
must be carefully controlled as it is a pivotal part of the
overall system security.
FIGURE 1-1:
CREATION AND STORAGE OF CRYPT KEY DURING PRODUCTION
The synchronization counter is the basis behind the
transmitted code word changing for each transmission;
it increments each time a button is pressed. Each incre-
ment of the synchronization value results in more than
50% of the hopping code bits changing.
Figure 1-2 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
crypt key are input to the encryption algorithm and the
output is 32 bits of encrypted information. This data will
change with every button press while its value will
appear to ‘randomly hop around’. Hence, this data 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 4.1.
A receiver may use any type of controller as a decoder.
Typically, it is a microcontroller with compatible firm-
ware that allows the decoder to operate in conjunction
with an HCS370 based transmitter.
A transmitter must first be ‘learned’ by the receiver
before its use is allowed in the system. Learning
includes calculating the transmitter’s appropriate crypt
key, decrypting the received hopping code, storing the
serial number, storing the synchronization counter
value, and storing crypt 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 the serial num-
ber is from a learned transmitter, the message is
decrypted and the synchronization counter is verified.
Finally, the button status is checked to see what opera-
tion is requested. Figure 1-3 shows the relationship
between some of the values stored by the receiver and
the values received from the transmitter.
For detailed decoder operation, see Section 7.0.
Transmitter
Manufacturer’s
Serial Number
Code
Crypt
Key
Key
Generation
Algorithm
Serial Number
Crypt Key
Sync Counter
.
.
.
HCS370
Production
Programmer
EEPROM Array


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