Another factor in system security is the control of the encoder. If attackers
gain control of the encoder, typically they would be able to access the
system. The HS offers the option of adding a Personal Identification
Number (PIN) to the encoder that must be entered before the encoder will
activate. 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 will calculate 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 will
logically invert all of the bits. This ensures that every packet will always
contain 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 will set the baud rate and go 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 will begin 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 will replicate the Data byte on its data lines and pull
the MODE_IND line high. It will also output a number that represents the ID
of the encoder. It will output this number once when the first valid packet is
received. The decoder will then look for the next valid packet. If an error is
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