HCS201 datasheet(11/36 Pages) MICROCHIP

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DS41098D-page 11

HCS201

5.0

SPECIAL FEATURES

5.1

Code Word Completion

The code word completion feature ensures that entire

code words are transmitted, even if the button is

released before the code word is complete. If the but-

ton is held down beyond the time for one code word,

multiple code words will result. If another button is acti-

vated during a transmission, the active transmission

will be aborted and a new transmission will begin using

the new button information.

5.2

VLOW: Voltage LOW Indicator

The VLOW bit is transmitted with every transmission

(Figure 9-4) and will be transmitted as a one if the

operating voltage has dropped below the low voltage trip

point. The trip point is selectable based on the battery

voltage being used. See Section 3.7.2 for a description

of how the low voltage select option is set. This VLOW

signal is transmitted so the receiver can give an audible

signal to the user that the transmitter battery is low.

5.3

Auto-Shutoff

The auto-shutoff function automatically stops the device

from transmitting if a button inadvertently gets pressed

for a long period of time. This will prevent the device from

draining the battery if a button gets pressed while the

transmitter is in a pocket or purse. Time-out period is TTO.

5.4

Seed Transmission

In order to increase the level of security in a system, it

is possible for the receiver to implement what is known

as a secure learn function. This can be done by utilizing

the seed value stored in EEPROM, transmitted only

when all three button inputs are pressed at the same

time (Table 5-1). Instead of the normal key generation

inputs being used to create the crypt key, this seed

value is used.

TABLE 5-1:

PIN ACTIVATION TABLE

5.5

Blank Alternate Code Word

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

duty cycle of the individual bits as well as by blanking

out consecutive code words. Blank Alternate Code

Word (BACW) may be used to reduce the average

power of a transmission by transmitting only every sec-

ond code word (Figure 5-1). This is a selectable feature

that is determined in conjunction with the baud rate

selection bit BSL0.

Enabling the BACW option may likewise allow the user

to transmit a higher amplitude transmission as the time

averaged power is reduced. BACW 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 5-1:

BLANK ALTERNATE CODE WORD (BACW)

Function

Standby

Hopping Code

Seed Code

Code Word

BRS = 0

BRS = 1

Time

Code Word

Amplitude

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