PIC12F635 datasheet(81/196 Pages) MICROCHIP

81 / 196 page

Preliminary

DS41232B-page 79

PIC12F635/PIC16F636/639

11.0

ANALOG FRONT-END (AFE)

FUNCTIONAL DESCRIPTION

(PIC16F639 ONLY)

The PIC16F639 device consists of the PIC16F636

device and low frequency (LF) Analog Front-End

(AFE), with the AFE section containing three analog-

input channels for signal detection and LF talk-back.

This section describes the Analog Front-End (AFE) in

detail.

The PIC16F639 device can detect a 125 kHz input

signal as low as 1 mVpp and transmit data by using

internal LF talk-back modulation or via an external

transmitter. The PIC16F639 can also be used for

various

bidirectional

communication

applications.

Figure 11-3 and Figure 11-4 show application examples

of the device.

Each analog input channel has internal tuning

capacitance, sensitivity control circuits, an input signal

strength limiter and an LF talk-back modulation

transistor. An Automatic Gain Control (AGC) loop is

used for all three input channel gains. The output of

each channel is OR'd and fed into a demodulator. The

digital output is passed to the LFDATA pin. Figure 11-1

shows the block diagram of the AFE and Figure 11-2

shows the LC input path.

There are a total of eight Configuration registers. Six of

them are used for AFE operation options, one for

column parity bits and one for status indication of AFE

operation. Each register has 9 bits including one row

parity bit. These registers are readable and writable by

SPI (Serial Protocol Interface) commands except for

the Status register, which is read-only.

11.1

RF Limiter

The RF Limiter limits LC pin input voltage by de-Q’ing

the attached LC resonant circuit. The absolute voltage

limit is defined by the silicon process’s maximum

allowed input voltage (see Section 15.0 “Electrical

Specifications”). The limiter begins de-Q’ing the

external LC antenna when the input voltage exceeds

VDE_Q

, progressively de-Q’ing harder to reduce the

antenna input voltage.

The signal levels from all 3 channels are combined

such that the limiter attenuates all 3 channels

uniformly, in respect to the channel with the strongest

signal.

11.2

Modulation Circuit

The modulation circuit consists of a modulation

transistor (FET), internal tuning capacitors and external

LC antenna components. The modulation transistor

and the internal tuning capacitors are connected

between the LC input pin and LCCOM pin. Each LC

input has its own modulation transistor.

When the modulation transistor turns on, its low Turn-on

Resistance (RM) clamps the induced LC antenna

voltage. The coil voltage is minimized when the

modulation transistor turns-on and maximized when the

modulation

transistor

turns-off.

The

modulation

transistor’s low Turn-on Resistance (RM) results in a

high modulation depth.

The LF talk-back is achieved by turning on and off the

modulation transistor.

The modulation data comes from the microcontroller

section via the digital SPI interface as “Clamp On”,

“Clamp Off” commands. Only those inputs that are

enabled will execute the clamp command. A basic

block diagram of the modulation circuit is shown in

Figure 11-1 and Figure 11-2.

The modulation FET is also shorted momentarily after

Soft Reset and Inactivity timer time-out.

11.3

Tuning Capacitor

Each channel has internal tuning capacitors for external

antenna tuning. The capacitor values are programmed

by the Configuration registers up to 63 pF, 1 pF per step.

11.4

Variable Attenuator

The variable attenuator is used to attenuate, via AGC

control, the input signal voltage to avoid saturating the

amplifiers and demodulators.

11.5

Sensitivity Control

The sensitivity of each channel can be reduced by the

channel’s Configuration register sensitivity setting.

This is used to desensitize the channel from optimum.

Note:

The user can control the tuning capacitor

by programming the AFE Configuration

registers.

Note:

The

variable

attenuator

function

accomplished by the device itself. The

user cannot control its function.

Note:

The user can desensitize the channel

sensitivity

programming

the

AFE

Configuration registers.

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