HCS473-SLROM datasheet(19/68 Pages) MICROCHIP

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 2002 Microchip Technology Inc.

Preliminary

DS40035C-page 17

HCS473

3.1.4.14

Guard Time Select (GSEL)

The guard time (T

G) select option determines the time

between consecutive code words when no data is

transmitted. The guard time may be selected in conjuc-

tion with the RF baud rate and preamble duty cycle to

control time-averaged power output for transmitter cer-

tification.

GSEL options:

•3 RF

• 6.4 ms

•51.2 ms

• 102.4 ms

3.1.4.15

Minimum Code Words (MTX)

The Minimum Code Words (MTX) configuration option

determines the minimum number of code words trans-

mitted when a momentary switch input is taken high for

more than T

PU, or when a proximity activation occurs.

MTX options:

•1 code word

•2 code words

•4 code words

•8 code words

3.1.4.16

Timeout Select (TSEL)

The HCS473’s Timeout function prevents battery drain

should a switch input remain high (stuck button) longer

than the selectable TSEL time. After the TSEL time, the

device will return to Low-power mode.

The device will stop transmitting in Low-power 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 if no button were stuck.

TSEL options:

•4s

•8s

•16s

•32s

3.1.4.17

Long Preamble Enable (LPRE)

Enabling the Long Preamble configuration option

extends the first code word’s preamble to a ‘long’ pre-

amble time LPRL

; allowing the receiver more time to

wake and bias before the data bits arrive. The longer

preamble will be a square wave at the selected RFT

Subsequent code words begin with the standard pre-

amble length.

LPRE options:

• Standard 16 high pulse preamble

• Long preamble, duration defined by LPRL

3.1.4.18

Long Preamble Length (LPRL)

The long preamble length option selects the first code

word’s preamble length when the long preamble option

(LPRE) is enabled. Only the first code word begins with

the long preamble, subsequent code words begin with

the standard 16 high pulses preamble.

LPRL options:

•75 ms

• 100 ms

3.1.4.19

Preamble Duty Cycle (PRD)

The preamble duty cycle can be set to either 33% or

50% to limit the average power transmitted, Figure 3-7.

PRD options:

• 50% Duty Cycle

• 33% Duty Cycle

FIGURE 3-7:

PREAMBLE FORMATS

3.1.5

LED OPERATION

The LED pin output varies depending on whether the

device V

DD is greater than VLOWSEL (a good battery) or

below V

LOWSEL (a flat battery).

The LED pin will periodically be driven low as long as

the device is transmitting and the battery is good. If the

supply voltage drops below the specified V

LOWSEL trip

point, the LED pin will be driven low only once for any

given device activation so long as the low battery con-

dition remains (Figure 3-8). If the battery voltage recov-

ers during the transmission, the LED will begin blinking

again.

E TE

E 2TE

33% Duty Cycle

50% Duty Cycle

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