HCS200
FIGURE 7-1: TYPICAL LEARN SEQUENCE
7.0
INTEGRATING THE HCS200
INTO A SYSTEM
Enter Learn
Mode
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.
Wait for Reception
of a Valid Code
Generate Key
from Serial Number
Use Generated Key
to Decrypt
7.1
Learning a transmitter to a receiver
Compare Discrimination
Value with Fixed Value
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.
No
Equal
?
Yes
Wait for Reception
of Second Valid Code
Use Generated Key
to Decrypt
Compare Discrimination
Value with Fixed Value
No
It must be stated that some learning strategies have
been patented and care must be taken not to infringe.
Equal
?
Yes
No
Counters
Sequential
?
Yes
Learn
Unsuccessful
Learn successful.
Store:
Serial number
Encryption key
Synchronization counter
Exit
DS40138A-page 10
Preliminary
1996 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
