HCS500_15 Datasheet, PDF (5/42 Pages) Microchip Technology – KEELOQ® Code Hopping 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 crypt key
• The current synchronization counter value
The decoder must also store the manufacturer’s code
(Section 1.1 “HCS Encoder Overview”) 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,
consequently, 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).
HCS500
3.2 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 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 possible to erase lost transmitters by
repeatedly learning new transmitters. To
remove lost or stolen transmitters,
ERASE_ALL transmitters and relearn all
available transmitters.
3: Learning a transmitter with a crypt key that
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.
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DS40000153E-page 5