HCS512_11 Datasheet, PDF (6/34 Pages) Microchip Technology – KEELOQ® Code Hopping Decoder system and high security

HCS512
4.0 DECODER OPERATION
4.1 Learning a Transmitter to a
Receiver
Either the serial number-based learning method or the
seed-based learning method can be selected. The
learning method is selected in the configuration byte. In
order for a transmitter to be used with a decoder, the
transmitter must first be ‘learned’. When a transmitter is
learned to a decoder, the decoder stores the crypt key,
a check value of the serial number and current syn-
chronization value in EEPROM. The decoder must
keep track of these values for every transmitter that is
learned. The maximum number of transmitters that can
be learned is four. The decoder must also contain the
Manufacturer’s Code in order to learn a transmitter.
The Manufacturer’s Code will typically be the same for
all decoders in a system.
The HCS512 has four memory slots. After an “erase all”
procedure, all the memory slots will be cleared. Erase
all is activated by taking LRNIN low for approximately
10 seconds. When a new transmitter is learned, the
decoder searches for an empty memory slot and stores
the transmitter’s information in that memory slot. When
all memory slots are full, the decoder randomly over-
writes existing transmitters.
4.1.1 LEARNING PROCEDURE
Learning is activated by taking the LRNIN input low for
longer than 64 ms. This input requires an external pull-
up resistor.
To learn a new transmitter to the HCS512 decoder, the
following sequence is required:
1. Enter Learning mode by pulling LRNIN low for
longer than 64 ms. The LRNOUT output will go
high.
2. Activate the transmitter until the LRNOUT out-
put goes low indicating reception of a valid code
(hopping message).
3. Activate the transmitter a second time until the
LRNOUT toggles for 4 seconds (in Secure
Learning mode, the seed transmission must be
transmitted during the second stage of learn by
activating the appropriate buttons on the trans-
mitter).
If LRNIN is taken low momentarily during the
learn status indication, the indication will be ter-
minated. Once a successful learning sequence
is detected, the indication can be terminated
allowing quick learning in a manufacturing
setup.
4. The transmitter is now learned into the decoder.
5. Repeat steps 1-4 to learn up to four transmitters.
6. Learning will be terminated if two non-sequential
codes were received or if two acceptable codes
were not decoded within 30 seconds.
DS40151E-page 6
The following checks are performed on the decoder to
determine if the transmission is valid during learn:
• The first code word is checked for bit integrity.
• The second code word is checked for bit integrity.
• The hopping code is decrypted.
• If all the checks pass, the serial number and syn-
chronization counters are stored in EEPROM
memory.
Figure 4-1 shows a flow chart of the learn sequence.
FIGURE 4-1: LEARN SEQUENCE
Enter Learn
Mode
Wait for Reception
of a Valid Code
Wait for Reception
of Second
Non-Repeated
Valid Code
Generate Key
from Serial Number
or Seed Value
Use Generated Key
to Decrypt
Compare Discrimination
Value with Serial Number
Equal
No
?
Yes
Learn successful. Store:
Serial number check value
Synchronization counter
crypt key
Learn
Unsuccessful
Exit
© 2011 Microchip Technology Inc.