MCS3142-ISLASHST datasheet(17/37 Pages) MICROCHIP

17 / 37 page

 2014 Microchip Technology Inc.

DS40001747A-page 17

MCS3142

4.5

Code Word Format

FIGURE 4-1:

CLASSIC KEELOQ® CODE WORD FORMAT

4.5.1

HOPPING CODE PORTION

The hopping code portion is calculated by encrypting

the synchronization counter, discrimination value and

function code with the encoder key. The hopping code

is calculated when a button press is registered.

4.5.2

FIXED CODE PORTION

The fixed code portion consists of 28 bits of the serial

number and a copy of the 4-bit function code. Two bits

of constant zero are prepended to the fixed code

portion.

4.5.3

SEED WORD FORMAT

A seed transmission transmits a code word that

consists of 60 bits of fixed data that is stored in the NVM

by the manufacturer. This can be used for secure

learning of encoders or whenever a fixed code

transmission is required. The seed code word format is

shown is Figure 4-2. The function code for a seed

transmission is always ‘1111’.

The

seed

word

transmitted

whenever

seed-configured button combination is registered. If the

Delayed option is enabled, the encoder will transmit 25

typical code words before transmitting seed words. If

the Limited option is enabled, the seed word will only

be transmitted if the encoder’s synchronization counter

is less than 256. If the synchronization counter is above

this, a typical code word will be transmitted instead.

FIGURE 4-2:

CLASSIC KEELOQ® SEED WORD FORMAT

Blan k

Ser ial Numbe r

Functio n

Code

DIS C

34 bits

Fixe d P ortion

Rev. 20-000 006A

8/29/201 3

Functio n

Code

Counter

Over

Flow

Sync

Counter

32 bits

Encrypted Portion

2-bits

4-bits

28-bits

4-bits

2-bits

10-bits

16-bits

Blank

Serial Number

Function

Code

6 bits

Fixed

Rev. 20-000008A

10/2/2013

60 bits

Seed

2-bits

4-bits

60-bits

Note:

In Seed code word, the fixed portion is sent as

0b001111.

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