Getting Started
STK512 User Guide
2-9
5170C–AVR–10/09
3.
Repeat steps 1 and 2 for to teach additional transmitters.
4.
When the last transmitter has been taught to the receiver, wait for the 10-second learn-mode timeout
to expire. At this Point, LED5 goes off and the receiver is ready to accept regular messages from the
transmitters.
Note:
Whenever the maximum number of transmitters is reached, LED5 blinks to indicate that a transmitter has
been learned, but there is not another 10-second delay. LED5 goes off immediately after blinking.
2.4
The Demonstration
On the STK500, LED0 through LED4 represent a sequential circular counter (i.e., LEDs 0 through 4 illu-
minate in sequence and then LEDs 0 through 4 again illuminate in sequence, etc.). Pressing Switch 3 on
a learned transmitter key fob increments the code counter and the illuminated LED; pressing Switch 2,
the counter and the illuminated LED de-increments.
Since the rolling code is transparent (embedded in the transmitted message) to the end user of a sys-
tem, by default, the software provided in this starter kit is designed to simply demonstrate the reliable
receipt of transmissions originating from the transmitter key fobs.
A necessary component of this secure encryption algorithm is synchronization between the transmitter
and receiver. To demonstrate synchronization feature, a situation must be created to cause a transmitter
key fob’s counter to become out of sequence with the receiver’s respective counter. With the source
code provided, the window of acceptance defaults to a value of 100. That is, the transmitter key fob must
increment its counter 100 times above the count the receiver remembers for that transmitter key fob.
This can be done by moving the transmitter key fob out of range of the receiver and pressing either
Switch 2 or 3 more than 100 times. When the transmitter key fob is again brought within range of the
receiver; note that the receiver will not respond to that transmitter key fob because the counters are out
of sequence more than 100. This synchronization feature is further explained in section 3.1.1 of the
“AVR411: Secure Rolling Code Algorithm for Wireless Link Application Note” that is furnished on the
CDROM.
To simplify demonstrating the out-of-sync situation, the size of the “rolling window of acceptance” vari-
able can be reduced. This reduces the number of times the transmitter key fob switches must be
pressed to cause an out of sync condition.
Similar to the above approach, incremented counter values can be changed through manipulation of the
counter value variable directly in software. Atmel recommends appropriate variable monitoring capabili-
ties be available before attempting this approach.
Whichever approach is used, the transmitter key fob and receiver counter can be resynchronized by
having the receiver relearn the transmitter key fob.
