HCS515_11 Datasheet, PDF (5/38 Pages) Microchip Technology – KEELOQ® Code Hopping Decoder stand-alone decoder

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 decryption 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 HCS515 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
command (C3H).
3.1.1 LEARNING PROCEDURE
Learning is initiated by sending the
ACTIVATE_LEARN (D2H) command to the decoder.
The decoder acknowledges reception of the command
by pulling the data line high.
For the HCS515 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.)
3. The HCS515 will transmit a learn-status string,
indicating that the learn was successful.
4. The decoder has now learned the transmitter.
5. 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 possible to erase lost
transmitters by repeatedly learning
new transmitters. To remove lost or
stolen transmitters, ERASE_ALL
transmitters and relearn all available
transmitters.
HCS515
3: Learning a transmitter with an encoder
decryption key that is identical to a
transmitter already in memory
replaces the existing transmitter. In
practice, this means that all transmit-
ters should have unique encoder
decryption 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 decryption key is generated accord-
ing 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 Decryption Key
Sync. Counter Value
Learn
Unsuccessful
Exit
© 2011 Microchip Technology Inc.
DS40183E-page 5