MCS3142-ISLASHST datasheet(27/37 Pages) MICROCHIP

27 / 37 page

 2014 Microchip Technology Inc.

DS40001747A-page 27

MCS3142

8.0

INTEGRATING MCS3142 INTO

A SYSTEM

FIGURE 8-1:

TYPICAL DECODER

OPERATION

8.1

Decoder Operation

The decoder waits until a transmission is received. The

received serial number is compared to the EEPROM

table of learned transmitters to first determine if this

transmitter’s use is allowed in the system. If from a

paired transmitter, the transmission is decrypted using

the stored crypt key and authenticated via the

Discrimination bits for appropriate crypt key usage. If

the decryption is valid, the synchronization value is

evaluated (see Figure 8-1).

8.2

Synchronization with a Decoder

The KEELOQ technology includes a sophisticated

synchronization technique that does not require the

calculation and storage of future codes. The technique

securely blocks invalid transmission while providing

transparent

resynchronization

transmitters

inadvertently activated away from the receiver.

Figure 8-2

shows

three-partition,

rotating

Synchronization window. The size of each window is

optional but the technique is fundamental. Each time a

transmission is authenticated, the intended function is

executed and the transmission’s synchronization

counter value is stored in EEPROM. From the currently

stored counter value there is an initial Single Operation

Forward window of 16 codes. If the difference between

a received synchronization counter and the last stored

counter is within 16, the intended function will be

executed on a single button press and the new

synchronization counter will be stored. Storing the new

synchronization counter value effectively rotates the

entire Synchronization window.

A Double Operation (Resynchronization) window

further exists from the Single Operation window up to

32K code forward of the currently stored counter value.

It is referred to as Double Operation because a

transmission with a synchronization counter in this

window will require an additional, sequential counter

transmission prior to executing the intended function.

Upon receiving the sequential transmission the

decoder executes the intended function and stores the

synchronization counter value. This resynchronization

occurs transparently to the user, as it is human nature

to press the button a second time if the first was

unsuccessful.

The third window is a Blocked window ranging from the

Double Operation window to the currently stored

synchronization counter value. Any transmission with

synchronization counter value within this window will

be ignored. This window excludes previously used

code-grabbed transmissions from accessing the

system.

Rev. 20-000013A

1/29/2014

Start

Does

Serial

Number

Match?

Transmission

Received?

Is Counter

Within 16?

Is Counter

Within 32K?

Decrypt Transmission

Decryption

Valid?

Save Counter in

Temporary Location

Execute

Command and

Update Counter

Yes

Note:

The synchronization method described in

this section is an exemplar method. It may

be altered to fit the needs and capabilities

of a particular system.

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