HCS500 datasheet(18/40 Pages) MICROCHIP

18 / 40 page

HCS500

DS40153D-page 18

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 66/69 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 hop-

ping 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 66/69-bit code word

when a button is pressed. The 66/69-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:

TRANSMISSION FORMAT (PWM)

FIGURE 7-2:

CODE WORD ORGANIZATION

LOGIC "1"

Guard

Time

50%

Encrypted

Portion

Fixed Code

Portion

LOGIC "0"

Preamble

Header

10xTE

TBP

Repeat

(1-bit)

VLOW

(1-bit)

Button

Status

S2 S1 S0 S3

Serial Number

(28 bits)

Button

Status

S2 S1 S0 S3

OVR

(2 bits)

DISC

(10 bits)

Sync Counter

(16 bits)

Repeat

(1-bit)

VLOW

(1-bit)

Button

Status

1 1 1 1

Serial Number

(28 bits)

SEED

(32 bits)

34 bits of Fixed Portion

32 bits of Encrypted Portion

66 Data bits

Transmitted

LSb first.

LSb

MSb

LSb

SEED replaces Encrypted Portion when all button inputs are activated at the same time.

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