HCS200-ISN MICROCHIP [Microchip Technology], HCS200-ISN Datasheet - Page 10

• Manufacturer Part Number: HCS200-ISN
• Description: KEELOQ CODE HOPPING ENCODER
• Manufacturer: MICROCHIP [Microchip Technology]

HCS200
7.0
Use of the HCS200 in a system requires a compatible
decoder. This decoder is typically a microcontroller with
compatible firmware. Microchip will provide (via a
license agreement) firmware routines that accept
transmissions from the HCS200 and decrypt the
hopping code portion of the data stream. These
routines provide system designers the means to
develop their own decoding system.
7.1
In order for a transmitter to be used with a decoder, the
transmitter must first be ‘learned’. Several learning
strategies can be followed in the decoder implementa-
tion. When a transmitter is learned to a decoder, it is
suggested that the decoder stores the serial number
and current synchronization value in EEPROM. The
decoder must keep track of these values for every
transmitter that is learned (Figure 7-1). The maximum
number of transmitters that can be learned is only a
function of how much EEPROM memory storage is
available. The decoder must also store the manufac-
turer’s code in order to learn a transmission transmitter,
although 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 is also better for security reasons.
It must be stated that some learning strategies have
been patented and care must be taken not to infringe.
DS40138A-page 10
INTEGRATING THE HCS200
INTO A SYSTEM
Learning a transmitter to a receiver
Preliminary
FIGURE 7-1:
Compare Discrimination
Compare Discrimination
Learn successful.
Store:
Value with Fixed Value
Value with Fixed Value
of Second Valid Code
from Serial Number
Use Generated Key
Use Generated Key
Serial number
Encryption key
Synchronization counter
Wait for Reception
Wait for Reception
of a Valid Code
Generate Key
Enter Learn
Sequential
to Decrypt
to Decrypt
Counters
Mode
Equal
Equal
Exit
?
?
?
TYPICAL LEARN SEQUENCE
Yes
Yes
Yes
1996 Microchip Technology Inc.
No
No
No
Unsuccessful
Learn