HCS500 Datasheet, PDF (5/24 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER

3.0 DECODER OPERATION
3.1 Learning a Transmitter to a Receiver
(Normal or Secure Learn)
Before the transmitter and receiver can work together,
the receiver must first ‘learn’ and store the following
information from the transmitter in EEPROM:
• A check value of the serial number
• The encoder key
• The current synchronization counter value
The decoder must also store the manufacturer’s code
(Section 1.2) in protected memory. This code will
typically be the same for all of the decoders in a system.
The HCS500 has seven memory slots, and, conse-
quently, can store up to seven transmitters. During the
learn procedure, the decoder searches for an empty
memory slot for storing the transmitter’s information.
When all of the memory slots are full, the decoder will
overwrite the last transmitter’s information. To erase all
of the memory slots at once, use the ERASE_ALL com-
mand (C3H).
3.1.1 LEARNING PROCEDURE
Learning is initiated by sending the ACTIVATE_LEARN
(D2H) command to the decoder. The decoder acknowl-
edges reception of the command by pulling the data
line high.
For the HCS500 decoder to learn a new transmitter, the
following sequence is required:
1. Activate the transmitter once.
2. Activate the transmitter a second time. (In
secure learning mode, the seed transmission
must be transmitted during the second stage of
learn by activating the appropriate buttons on
the transmitter.)
The HCS500 will transmit a learn-status string,
indicating that the learn was successful.
3. The decoder has now learned the transmitter.
4. Repeat steps 1-3 to learn up to seven
transmitters
Note 1: Learning will be terminated if two
nonsequential codes were received or if two
acceptable codes were not decoded within
30 seconds.
2: If more than seven transmitters are learned,
the new transmitter will replace the last
transmitter learned. It is, therefore, not pos-
sible to erase lost transmitters by
repeatedly learning new transmitters. To
remove lost or stolen transmitters,
ERASE_ALL transmitters and relearn all
available transmitters.
HCS500
3: Learning a transmitter with an encoder key
that is identical to a transmitter already in
memory replaces the existing transmitter. In
practice, this means that all transmitters
should have unique encoder keys. Learning
a previously learned transmitter does not
use any additional memory slots.
The following checks are performed by 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 encoder key is generated according to the
selected algorithm.
• The hopping code is decrypted.
• The discrimination value is checked.
• If all the checks pass, the key, serial number
check value, and synchronization counter values
are stored in EEPROM memory.
Figure 3-1 shows a flow chart of the learn sequence.
FIGURE 3-1: 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/
Seed Value
Use Generated Key
to Decrypt
Compare Discrimination
Value with Serial Number
Equal? ^No
Yes
Learn successful. Store:
Serial number check value
Encoder key
Sync. counter value
Learn
Unsuccessful
Exit
© 1997 Microchip Technology Inc.
Preliminary
DS40153B-page 5