HCS300

DS21137F-page 8

© 2001 Microchip Technology Inc.

4.0

TRANSMITTED WORD

4.1

Code Word Format

The HCS300 code word is made up of several parts

(Figure 4-1). Each code word contains a 50% duty

cycle preamble, a header, 32 bits of encrypted data and

34 bits of fixed data followed by a guard period before

another code word can begin. Refer to Table 8-4 for

code word timing.

4.2

Code Word Organization

The HCS300 transmits a 66-bit code word when a

button is pressed. The 66-bit word is constructed from

a Fixed Code portion and an Encrypted Code portion

(Figure 4-2).

The 32 bits of Encrypted Data are generated from 4

button bits, 12 discrimination bits and the 16-bit sync

value. The encrypted portion alone provides up to four

billion changing code combinations.

The 34 bits of Fixed Code Data are made up of 2 sta-

tus bits, 4 button bits and the 28-bit serial number. The

fixed and encrypted sections combined increase the

FIGURE 4-1:

CODE WORD FORMAT

FIGURE 4-2:

CODE WORD ORGANIZATION

LOGIC ‘0’

LOGIC ‘1’

Bit

Period

Preamble

Header

Encrypted Portion

of Transmission

Fixed Portion of

Transmission

Guard

Time

50% Duty Cycle

Repeat

(1 bit)

(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)

(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

MSb

LSb

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