HCS412 datasheet(13/44 Pages) MICROCHIP

13 / 44 page

Preliminary

DS41099C-page 13

HCS412

FIGURE 3-5:

PWM TRANSMISSION FORMAT—MOD = 0

FIGURE 3-6:

MANCHESTER TRANSMISSION FORMAT—MOD = 1

3.4

Encoder Special Features

3.4.1

CODE WORD COMPLETION AND

MINIMUM CODE WORDS

The code word completion feature ensures that entire

code words are transmitted, even if the active button is

released before the code word transmission is com-

plete. If the button is held down beyond the time for one

code word, multiple complete code words will result.

The device default is that a momentary button press

will transmit at least one complete code word. Enable

the Minimum Four Code Words (MTX4) configuration

option to extend this feature such that a minimum of 4

code words are completed on a momentary button acti-

vation.

3.4.2

AUTO-SHUTOFF

The Auto-shutoff function prevents battery drain should

a button get stuck for a long period of time. The time

period (TTO) is approximately 20 seconds, after which

the device will enter Time-out mode.

The device will stop transmitting in Time-out mode but

there will be leakage across the stuck button input’s

internal pull-down resistor. The current draw will there-

fore be higher than when in Standby mode.

3.4.3

CODE WORD BLANKING ENABLE

Federal Communications Commission (FCC) part 15

rules specify the limits on worst case average funda-

mental power and harmonics that can be transmitted in

a 100 ms window. For FCC approval purposes, it may

therefore be advantageous to minimize the transmis-

sion duty cycle. This can be achieved by minimizing the

on-time of the individual bits as well as by blanking out

consecutive code words.

TOTAL TRANSMISSION:

Preamble Sync Encrypt

Fixed

Guard

1 CODE WORD

Preamble Sync

Encrypt

LOGIC "1"

Guard

Time

31 RFTE Preamble, 50% Duty Cycle

Encrypted

Portion

Fixed Code

Portion

LOGIC "0"

10TE

Header

CODE WORD

Long Preamble (LPRE) disabled

TOTAL TRANSMISSION:

Sync

Encrypt

Fixed

Guard

1 CODE WORD

Preamble

Sync

Encrypt

Preamble

CODE WORD

Guard

50% Duty

Header

Encrypted

Fixed Code

START bit

STOP bit

Time

Portion

bit 0 bit 1 bit 2

LOGIC "0"

LOGIC "1"

Preamble

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