HCS412SN datasheet(7/44 Pages) MICROCHIP

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Preliminary

DS41099C-page 7

HCS412

TABLE 2-2:

OSCILLATOR CALIBRATION

VALUE (OSCT)

FIGURE 2-7:

HCS412 NORMALIZED RFTE

VERSUS TEMP

2.2.6

LOW VOLTAGE DETECTOR

The HCS412’s battery voltage detector detects when

the supply voltage drops below a predetermined value.

The value is selected by the Low Voltage Trip Point

Select (VLOWSEL) configuration option.

The low voltage detector result is included in encoder

transmissions (VLOW) allowing the receiver to indicate

when the transmitter battery is low (Figure 3-2).

The HCS412 indicates a low battery condition by

changing the LED operation (Figure 3-9).

FIGURE 2-8:

TYPICAL VOLTAGE TRIP

POINTS

TABLE 2-3:

VLOWSEL OPTIONS

TABLE 2-4:

VLOW STATUS BIT

2.2.7

THE S2/RFEN/LC1 PIN

The S2/RFEN/LC1 pin may be used as a button input,

RF enable output or as an interface to the LF antenna.

Select between LC1 antenna interface and S2/RFEN

functionality with the button/transponder select (S2LC)

configuration option (Table 2-2).

2.2.7.1

S2 BUTTON INPUT CONSIDERATIONS

The S2/RFEN/LC1 pin defaults to LF antenna output

LC1 when the HCS412 is first connected to the supply

voltage (i.e., battery replacement).

The configuration register controlling the pin’s function

is loaded on the first device activation after battery

replacement. A desired S2 input state is therefore

enabled only after the first activation of either S0, S1 or

LC0. The transponder bias circuitry switches off and

the internal pull-down resistor is enabled when the S2/

RFEN/LC1 pin reaches button input configuration.

There will be an extra delay the first activation after

connecting to the supply voltage while the HCS412

retrieves the configuration word and configures the

pins accordingly.

OSCT3:0

Description

0111b

Slowest Oscillator Setting (long TE)

0011b

0010b

0001b

Slower (longer TE)

0000b

Nominal Setting

1111b

1110b

1101b

Faster (shorter TE)

1000b

Fastest Oscillator Setting (short TE)

0.94

1.10

1.08

1.06

1.04

1.02

1.00

0.98

0.96

0.92

0.90

RFTE

VDD LEGEND

= 2.0V

= 3.0V

= 6.0V

NORMALIZED

Temperature °C

-50 -40 -30 -20 -10 0

10 20 30 40 50 60 70 80 90

Note:

Values are for calibrated oscillator.

RFTE

VLOWSEL

Nominal

Trip

Point

Description

2.2V

for 3V battery applications

4.4V

for 6V battery applications

VLOW

Description

0VDD is above selected trip voltage

1VDD is below selected trip voltage

VLOW

Volts (V)

-40

050

2.0

1.6

1.8

2.2

2.4

2.6

Temp (°C)

VLOW sel = 0

4.4

4.0

4.2

3.8

4.6

4.8

5.0

VLOW sel = 1

2.8

Nominal VLOW trip point

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