hcs500 Microchip Technology Inc., hcs500 Datasheet - Page 5

• Manufacturer Part Number: hcs500
• Description: Keeloq Code Hopping Decoder
• Manufacturer: Microchip Technology Inc.

3.0
3.1
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 crypt key
• The current synchronization counter value
The decoder must also store the manufacturer’s code
(Section 1.1) in protected memory. This code will
typically be the same for all of the decoders in a sys-
tem.
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).
3.2
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.
2.
3.
4.
Note 1: Learning
2002 Microchip Technology Inc.
Activate the transmitter once.
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.
The decoder has now learned the transmitter.
Repeat steps 1-3 to learn up to seven
transmitters
2: If more than seven transmitters are
DECODER OPERATION
Learning a Transmitter to a
Receiver (Normal or Secure Learn)
LEARNING PROCEDURE
nonsequential codes were received or if
two acceptable codes were not decoded
within 30 seconds.
learned, the new transmitter will replace
the last transmitter learned. It is, therefore,
not possible to erase lost transmitters by
repeatedly learning new transmitters. To
remove
ERASE_ALL transmitters and relearn all
available transmitters.
lost
will
or
be
stolen
terminated
transmitters,
if
two
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 crypt key is generated according to the
• The hopping code is decrypted.
• The discrimination value is checked.
• If all the checks pass, the key, serial number
Figure 3-1 shows a flow chart of the learn sequence.
FIGURE 3-1:
selected algorithm.
check value, and synchronization counter values
are stored in EEPROM memory.
Serial number check value
Value with Serial Number
Compare Discrimination
Learn successful. Store:
from Serial Number/
Use Generated Key
Sync. counter value
Wait for Reception
3: Learning a transmitter with a crypt key that
Wait for Reception
Non-Repeated
of a Valid Code
Generate Key
Seed Value
Enter Learn
is identical to a transmitter already in mem-
ory replaces the existing transmitter. In
practice, this means that all transmitters
should have unique crypt keys. Learning a
previously learned transmitter does not use
any additional memory slots.
of Second
Valid Code
to Decrypt
crypt key
Mode
Equal?
Exit
LEARN SEQUENCE
Yes
No
HCS500
Unsuccessful
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