HCS512-SN datasheet(13/20 Pages) MICROCHIP

13 / 20 page

HCS512

DS40151C-page 13

8.0

KEELOQ ENCODERS

8.1

Transmission Format (PWM)

The KEELOQ encoder transmission is made up of sev-

eral parts (Figure 8-1). Each transmission begins with

a preamble and a header, followed by the encrypted

and then the ﬁxed data. The actual data is 56/66/67 bits

which consists of 32 bits of encrypted data and 24/34/

35 bits of non-encrypted data.

Each transmission is

followed by a guard period before another transmission

can begin. The encrypted portion provides up to four

billion changing code combinations and includes the

button status bits (based on which buttons were acti-

vated) along with the synchronization counter value

and some discrimination bits. The non-encrypted por-

tion is comprised of the status bits, the function bits,

and the 24/28-bit serial number. The encrypted and

non-encrypted combined sections increase the number

of combinations to 7.38 x 1019.

8.2

Code Word Organization

The HCSXXX encoder transmits a 66/67-bit code word

when a button is pressed. The 66/67-bit word is con-

structed from an encryption portion and a non-

encrypted code portion (Figure 8-2).

The Encrypted Data is generated from four button bits,

two overﬂow counter bits, ten discrimination bits, and

the 16-bit synchronization value.

The Non-encrypted Data is made up from 2 status

bits, 4 function bits, and the 28/32-bit serial number.

FIGURE 8-1:

CODE WORD TRANSMISSION FORMAT

FIGURE 8-2:

CODE WORD ORGANIZATION

LOGIC ‘0’

LOGIC ‘1’

Bit

Period

Preamble

Header

Encrypted Portion

of Transmission

Fixed Portion of

Transmission

Guard

Time

THOP

TFIX

Repeat

Vlow

(1 bit)

Button Sta-

tus (4 bits)

28-bit

Serial Number

Button

Status

(4 bits)

Discrimina-

tion bits

(12 bits)

16-bit

Sync Value

CRC1*

CRC0*

3/2 bits

Serial Number and But-

ton Status (32 bits)

32 bits of Encrypted Data

Encrypted Data

Non-encrypted Data

*HCS360/361

66/67 bits

of Data

Transmitted

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