HCS200-SLASHP datasheet(9/34 Pages) MICROCHIP

9 / 34 page

DS40138D-page 9

HCS200

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

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.

5.3

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.

5.4

VLOW: Voltage LOW Indicator

The VLOW signal is transmitted so the receiver can give

an indication to the user that the transmitter battery is

low. The VLOW bit is included in every transmission

(Figure 4-2 and Figure 9-5) and will be transmitted as a

zero if the operating voltage is above the low voltage

trip point. Refer to Figure 4-2. The trip point is select-

able based on the battery voltage being used. See Sec-

tion 3.6.3 for a description of how the low voltage trip

point is configured.

FIGURE 5-1:

BLANK ALTERNATE CODE WORD (BACW)

TABLE 5-1:

PIN ACTIVATION TABLE

External

Internal

Standby

000

Hopping Code

Transmission

100

000

100

011

111

011

Seed Transmission

111

BACW Disabled

(All words transmitted)

BACW Enabled

(1 out of 2 transmitted)

Transmission Amplitude = A

Code Word

Code

Word

Code

Word

Code

Word

Code

Word

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