HCS412_11 Datasheet, PDF(14/52 Page) Microchip Technology – KEELOQ® Code Hopping Encoder and Transponder Burglar alarm systems

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HCS412_11 Datasheet, PDF (14/52 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder and Transponder Burglar alarm systems

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HCS412

The Code Word Blanking Enable (CWBE) option may

be used to reduce the average power of a transmission

by transmitting only every second or every fourth code

word (Figure 3-7). This selectable feature is

determined in conjunction with the baud rate selection

bit RFBSL (Table 3-7).

Enabling the CWBE option may similarly allow the user

to transmit a higher amplitude transmission as the time

averaged power is reduced. CWBE effectively halves

the RF on-time for a given transmission so the RF out-

put power could theoretically be doubled while main-

taining the same time averaged output power.

FIGURE 3-7: CODE WORD BLANKING

RF Output Amplitude = A

CWBE Disabled

(All words transmitted)

Code Code

Word Word

CWBE Enabled

(1 out of 2 transmitted)

Code

Word

Code

Word

Code

Word

Code

Word

Code

Word

Code

Word

Code

Word

Code

Word

Code

Word

Code

Word

CWBE Enabled

(1 out of 4 transmitted)

Code

Word

Code

Word

TABLE 3-7:

RFBSL1:0

00b

01b

10b

11b

CODE WORD BLANKING ENABLE (CWBE)

CWBE

PWM RFTE Manchester RFTE

400 Î¼s

200 Î¼s

100 Î¼s

800 Î¼s

400 Î¼s

200 Î¼s

Transmit...

All code words

Every other code word

All code words

Every other code word

All code word

Every fourth code word

3.4.4 DELAYED INCREMENT (DINC)

The HCS412âs Delayed Increment feature advances

the synchronization counter by 12 a period of TTO after

the encoder activation occurs, for additional security.

The next activation will show a synchronization counter

increase of 13, not 1.

If the active button is released before the time-out TTO

has elapsed, the device stops transmitting but remains

powered for the duration of the time-out period. The

device will then advance the stored synchronization

counter by 12 before powering down.

If the active button is released before the time-out TTO

has elapsed and another activation occurs while wait-

ing out the time-out period, the time-out counter will

RESET and the resulting transmission will contain syn-

chronization counter value +1.

Note: If delayed increment is enabled, the QUE

counter will not reset to 0 until timeout TTO

has elapsed.

3.4.5 PLL INTERFACE

If the RFEN/S2/LC1 pin is configured as an RF enable

output, the pinâs behavior is coordinated with the DATA

pin to enable a typical PLLâs ASK or FSK mode.

The PLL Interface (AFSK) configuration option controls

the output as shown in Figure 3-8.

TABLE 3-8: PLL INTERFACE(AFSK)

AFSK

Description

ASK PLL Setup

FSK PLL Setup

3.4.6 LED OUTPUT

During normal operation (good battery), while transmit-

ting data the deviceâs LED pin will periodically be driven

low as indicated in Figure 3-9.

If the supply voltage drops below the trip point specified

by VLDWSEL, the LED pin will be driven low only once

for a longer period of time.

3.4.7 LONG PREAMBLE (LPRE)

Enabling the Long Preamble configuration option

extends the first code wordâs 50% duty cycle preamble

to a âlongâ preamble time TLPRE. The longer preamble

will be a square wave at the selected RFTE

(Figure 3-10).

DS41099D-page 14

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