PIC12F675T-ESLASHP datasheet(43/136 Pages) MICROCHIP

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DS41190G-page 43

PIC12F629/675

7.0

ANALOG-TO-DIGITAL

CONVERTER (A/D) MODULE

(PIC12F675 ONLY)

The

Analog-to-Digital

converter

(A/D)

allows

conversion of an analog input signal to a 10-bit binary

representation of that signal. The PIC12F675 has four

analog inputs, multiplexed into one sample and hold

circuit. The output of the sample and hold is connected

to the input of the converter. The converter generates a

binary result via successive approximation and stores

the result in a 10-bit register. The voltage reference

used in the conversion is software selectable to either

VDD or a voltage applied by the VREF pin. Figure 7-1

shows the block diagram of the A/D on the PIC12F675.

FIGURE 7-1:

A/D BLOCK DIAGRAM

7.1

A/D Configuration and Operation

There are two registers available to control the

functionality of the A/D module:

ADCON0 (Register 7-1)

ANSEL (Register 7-2)

7.1.1

ANALOG PORT PINS

The ANS3:ANS0 bits (ANSEL<3:0>) and the TRISIO

bits control the operation of the A/D port pins. Set the

corresponding TRISIO bits to set the pin output driver

to its high-impedance state. Likewise, set the

corresponding ANS bit to disable the digital input

buffer.

7.1.2

CHANNEL SELECTION

There are four analog channels on the PIC12F675,

AN0

through

AN3.

The

CHS1:CHS0

bits

(ADCON0<3:2>) control which channel is connected to

the sample and hold circuit.

7.1.3

VOLTAGE REFERENCE

There are two options for the voltage reference to the

A/D converter: either VDD is used, or an analog voltage

applied to VREF is used. The VCFG bit (ADCON0<6>)

controls the voltage reference selection. If VCFG is set,

then the voltage on the VREF pin is the reference;

otherwise, VDD is the reference.

7.1.4

CONVERSION CLOCK

The A/D conversion cycle requires 11 TAD. The source

of the conversion clock is software selectable via the

ADCS bits (ANSEL<6:4>). There are seven possible

clock options:

•FOSC/2

•FOSC/4

•FOSC/8

•FOSC/16

•FOSC/32

•FOSC/64

•FRC (dedicated internal RC oscillator)

For correct conversion, the A/D conversion clock

(1/TAD) must be selected to ensure a minimum TAD of

1.6

s. Table 7-1 shows a few TAD calculations for

selected frequencies.

GP0/AN0

ADC

GP1/AN1/VREF

GP2/AN2

GP4/AN3

VDD

VREF

ADON

GO/DONE

VCFG =

CHS1:CHS0

ADRESH ADRESL

ADFM

VSS

Note:

Analog voltages on any pin that is defined

as a digital input may cause the input

buffer to conduct excess current.

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