HCS500T-ISLASHST datasheet(19/42 Pages) MICROCHIP

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DS40000153E-page 19

HCS500

7.0

KEELOQ ENCODERS

7.1

Transmission Format (PWM)

The KEELOQ encoder transmission is made up of

several 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 followed 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

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

constructed 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 two Status

bits, four 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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