HCS473T/SL Microchip Technology, HCS473T/SL Datasheet - Page 41

• Manufacturer Part Number: HCS473T/SL
• Description: IC KEELOQ 3AXIS TRANSCODR 14SOIC
• Manufacturer: Microchip Technology
• Series: KEELOQ®r
• Type: Code Hopping Encoder and Transponderr

Specifications of HCS473T/SL

• Applications: Access Control Systems
• Mounting Type: Surface Mount
• Package / Case: 14-SOIC (3.9mm Width), 14-SOL
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

5.0
Use of the HCS473 in a system requires a compatible
decoder. This decoder is typically a microcontroller with
a low frequency coil antenna and radio frequency
receiver. Example firmware routines that accept and
decrypt K
tion Notes and the K
5.1
In order for a transmitter to be used with a decoder, the
transmitter must first be ‘learned’. When a decoder
learns a transmitter, it is suggested that the decoder
stores the serial number and current synchronization
value in EEPROM. Some learning strategies have
been patented and care must be taken not to infringe
on them. The decoder must keep track of these values
for every transmitter that is learned (see Figure 5-1).
The maximum number of transmitters that can be
learned is limited only by the available EEPROM
memory. The decoder must also store the manufac-
turer’s code in order to learn a transmitter. This value
will not change in a typical system, so it is usually
stored as part of the microcontroller ROM code. Storing
the manufacturer’s code as part of the ROM code
improves security by keeping it off the external bus to
the EEPROM.
 2002 Microchip Technology Inc.
INTEGRATING THE HCS473
INTO A SYSTEM
Training the Receiver
EE
L
OQ
transmissions can be found in Applica-
EE
L
OQ
license disk.
Preliminary
FIGURE 5-1:
5.2
In a typical decoder operation (Figure 5-2), the key
generation on the decoder side is performed by taking
the serial number from a transmission and combining
that with the manufacturer’s code to create the same
secret key that was used by the transmitter. Once the
secret key is obtained, the rest of the transmission can
be decrypted. The decoder waits for a transmission
and immediately can check the serial number to
determine if it is a learned transmitter. If it is, the
encrypted portion of the transmission is decrypted
using the stored key. It uses the discrimination bits to
determine if the decryption was valid. If everything up
to this point is valid, the synchronization value
is evaluated.
Synchronization Counter
Compare Discrimination
Learn Successful Store:
of a Valid Code & Seed
Value with Fixed Value
of Second Valid Code
Use Generated Key
Wait for Reception
Decoder Operation
Wait for Reception
Serial Number
Generate Key
Encoder Key
Enter Learn
Sequential
to Decrypt
(Optional)
Counters
Mode
Equal
Exit
?
?
Yes
Yes
TYPICAL LEARN
SEQUENCE
No
No
HCS473
DS40035C-page 39
Unsuccessful
Learn