Linx Technologies LICAL-DEC-HS001 - Manual (Page 9)

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encoder that must be entered before the encoder activates. Furthermore,
since each encoder has its own key and the Control Permissions are stored
in the decoder, all the attackers would be able to do is duplicate the device
that they have already taken. They will not be able to grant themselves
greater authority, create a new controller, or replicate another encoder.
Before the encoder sends a packet, it calculates the Hamming Weight (the
number of ‘1’s in the string) of the packet to determine the duty cycle. If the
duty cycle is greater than 50% (more ‘1’s than ‘0’s), the encoder logically
inverts all of the bits. This ensures that every packet always contains 50%
or less ‘1’s. Since the FCC allows transmitter output power to be averaged
over 100ms, this allows a legal improvement in link range and performance
for many devices using an ASK / OOK transmitter. A 50% duty cycle is
generally the best compromise between data volume and output power.
Some other manufacturers may use a Pulse Width Modulation (PWM)
scheme or Manchester Encoding scheme to maintain a 50% duty cycle.
Both of these methods work, but are inefficient and do not make use of
the full link budget. The HS Series uses true serial data while maintaining a
50% duty cycle. Application Note AN-00310 covers these issues in detail.
Decoder Power-Up
When the decoder first powers up, it sets the baud rate and goes to sleep
until: 1) the LEARN line is taken high, placing the decoder into Learn Mode,
2) a rising edge (low to high transition) on the COPY_IN line puts it into Get
Copy Mode, or 3) a rising edge on the DATA_IN line puts it into Receive
Mode.
Decoder Receive Mode
When a rising edge is seen on the DATA_IN line, the decoder enters
Receive Mode. It begins by looking for a valid packet (meaning one that
can be decrypted with the saved key) that has no errors. If the packet is
valid, then the decoder replicates the Data byte on its data lines and pulls
the MODE_IND line high. It also outputs a number that represents the ID of
the encoder once when the first valid packet is received. The decoder then
looks for the next valid packet. If an error is detected at any time, or if the
transmission cannot be decrypted with the saved key, then the decoder
ignores the packet and looks for the next one.
If no valid packet is detected within 262ms, the decoder goes back to
sleep.
Decoder Create Key Mode
Create Key Mode is entered when the CREATE_KEY line is high when
the LEARN line is taken high. When this happens, the MODE_IND line
goes high as an indication that the decoder is ready to create the key. The
CREATE_KEY line needs to go high ten times to set the key. Each edge
on the line starts a timer that is used to populate a part of the key. This
method is used to gather entropy so that the key is truly random and is
chosen from among all 2
80 possible keys.
Following the tenth press, the decoder begins to send the key to the
encoder on the KEY_OUT line. This is output as a serial data stream, so it
can be sent to the encoder by any method suitable for serial data transfer.
This can include the use of a wire, contact points on an enclosure, or
infrared. The HS Series Master Development System demonstrates wire
and infrared transfer methods. Please refer to the development system
User’s Guide for circuit schematics and further details.
Once the encoder receives the key on its KEY_IN line, it sends a
confirmation to the decoder through its DATA_OUT line. This means that
the standard mode of communication, whether a wire, RF, or infrared,
must be active. When the decoder receives this confirmation, it sends a
final confirmation through the KEY_OUT line. The MODE_IND LED lines on
the encoder and the decoder turn on for one second. This indicates that
the encoder and decoder are now ready to be used. The decoder outputs
the key information for seventeen seconds or until it receives a valid
confirmation from the encoder. If Control Permissions are going to be used,
they may now be set as described in the Decoder Learn Mode section.
Note that the CREATE_KEY line should be connected to a button or
another contact that will give random times between presses. Connecting
this line to a deterministic source, such as a microprocessor clock, does
not produce a secure key and could compromise the system.