MCS3142 datasheet(42/57 Pages) MICROCHIP

42 / 57 page

PC Application

 2014 Microchip Technology Inc.

DS40001746A-page 41

Classic KEELOQ

The Counter Delta box indicates the difference between two transmitters’ sync counter

values. Since the graphic updates only to the latest transmitter received, this value is

sometimes invalid until at least two transmissions from a single device are performed.

The red circle represents a simplified Synchronization window as shown in Figure 8-8.

The size of the pie is arbitrary, but the technique is fundamental.

FIGURE 8-8:

SYNCHRONIZATION WINDOW FOR THE CLASSIC KEELOQ

ALGORITHM

The synchronization counter is an always-incrementing event counter. It increments

whenever a new packet is prepared on the transmitter. Each time a transmission is

authenticated, the intended function is executed and the transmission’s synchroniza-

tion counter value is stored in EEPROM. From the currently stored counter value there

is an initial “Single Operation” forward window of 16 codes. This 16-code “window” is

represented by the dotted blue line. The current sync value is represented by the black

line and the previous received by the blue line.

FIGURE 8-9:

COUNTER DELTA 1

If the difference between a received synchronization counter and the last stored

counter is within 16, the intended function will be executed on the single button press

and the new synchronization counter will be stored. Storing the new synchronization

counter value effectively rotates the entire Synchronization window.

Note: The proportional aspects of the pie have been manipulated so that it is

easier to view the fundamental operation

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