HCS412SN datasheet(8/44 Pages) MICROCHIP

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HCS412

DS41099C-page 8

Preliminary

2.2.7.2

TRANSPONDER INTERFACE

Connecting an LC resonant circuit between the LC0

and the LC1 pins creates the bi-directional low fre-

quency communication path with the HCS412.

The internal circuitry on the HCS412 provides the fol-

lowing functions:

• LF input amplifier and envelope detector to detect

and shape the incoming low frequency excitation

signal.

• 10V zener input protection from excessive

antenna voltage generated when proximate to

very strong magnetic fields.

• LF antenna clamping transistors for inductive

responses back to the transponder reader. The

antenna ends are shorted together, ‘clamped’,

dissipating the oscillatory energy. The reader

detects this as a momentary load on its excitation

antenna.

• Damping circuitry that improves communication

when using high-Q LC antenna circuits.

• Incoming LF energy rectification and regulation

for the supply voltage in battery-less or low bat-

tery transponder instances.

During normal transponder operation, the LC1 pin func-

tions to bias the LC0 AGC amplifier input. The amplifier

gain control sets the optimum level of amplification in

respect to the incoming signal strength. The signal then

passes through an envelope detector before interpreta-

tion in the logic circuit.

2.2.7.3

RF ENABLE OUTPUT

When the RF enable (RFEN) configuration option is

enabled, the RFEN signal output is coordinated with

the DATA output pin to provide typical ASK or FSK PLL

activation.

TABLE 2-1:

RFEN OPTION

TABLE 2-2:

S2/RFEN/LC1 CONFIGURATION OPTION

3.0

ENCODER OPERATION

3.1

Encoder Activation

3.1.1

BUTTON ACTIVATION

The main way to enter Encoder mode is when the

wake-up circuit detects a button input activation; button

input transition from GND to VDD. The HCS412 control

logic wakes and delays a switch debounce time prior to

sampling the button inputs. The button input states,

cumulatively called the button status, determine

whether the HCS412 transmits a code hopping or seed

transmission, Table 3-1.

Additional button activations added during a transmis-

sion will immediately RESET the HCS412, perhaps

leaving the current code word incomplete. The device

will start a new transmission which includes the

updated button code value.

Buttons removed during a transmission will have no

effect unless no buttons remain activated. If no button

activations remain, the minimum number of compete

code words will be completed (Section 3.4.1) and the

device will return to Standby mode.

3.1.2

PROXIMITY ACTIVATION

The other way to enter Encoder mode is if the S2/LC

option is configured for LC operation and the wake-up

circuit detects a signal on the LC0 LF antenna input pin.

This form of activation is called Proximity activation as

a code hopping transmission would be initiated when

the device was proximate to a LF field.

Refer to Section 4.4 for details on configuring the

HCS412 for Proximity Activation.

RFEN

Description

RF Enable output is disabled.

RF Enable output is enabled.

S2LC

Resulting S2/RFEN/LC1 Configuration

• LC1 low frequency antenna output driver for inductive responses and LC bias.

Note: LC0 low frequency antenna input is also enabled.

• S2 button input pin with Schmitt Trigger detector and internal pull-down resistor.

• RFEN output driver.

Note: LC0 and LC1 low frequency antenna interfaces are disabled and the transponder circuitry is

switched off to reduce standby current.

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