HCS500-IP datasheet(17/24 Pages) MICROCHIP

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HCS500

Preliminary

DS40153B-page 17

7.0

KEELOQ ENCODERS

7.1

Transmission Format (PWM)

The KEELOQ encoder transmission is made up of sev-

eral parts (Figure 7-1). Each transmission begins with

a preamble and a header, followed by the encrypted

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

which consists of 32 bits of encrypted data and 34/35

bits of non-encrypted data.

Each transmission is fol-

lowed by a guard period before another transmission

can begin. The code hopping portion provides up to

four billion changing code combinations and includes

the button status bits (based on which buttons were

activated), along with the synchronization counter value

and some discrimination bits. The non-code hopping

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

and the 28-bit serial number. The encrypted and non-

encrypted combined sections increase the number of

combinations to 7.38 x 1019.

7.2

Code Word Organization

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

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

structed from a code hopping portion and a non-code

hopping portion (Figure 7-2).

The Encrypted Data is generated from four button bits,

two overﬂow counter bits, ten discrimination bits, and

the 16-bit synchronization counter value.

The Non-encrypted Data is made up from 2 status

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

FIGURE 7-1:

CODE WORD TRANSMISSION FORMAT

FIGURE 7-2:

CODE WORD ORGANIZATION

LOGIC ‘0’

LOGIC ‘1’

Bit

Period

Preamble

Header

Code Hopping Portion

of Transmission

Fixed Portion of

Transmission

Guard

Time

THOP

TFIX

Repeat

VLOW

(1 bit)

Button Status

S2S1S0S3

(4 bits)

28-bit

Serial

Number

Button Status

S2S1S0S3

(4 bits)

Discrimination

bits (12 bits)

16-bit

Sync.

Counter

Value

CRC1*

CRC0*

3/2 bits

Serial Number and

Button 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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