PIC16F720-ESLASHSS datasheet(78/244 Pages) MICROCHIP

78 / 244 page

PIC16F/LF720/721

DS41430A-page 78

 2010 Microchip Technology Inc.

9.1

ADC Configuration

When configuring and using the ADC the following

functions must be considered:

• Port configuration

• Channel selection

• ADC conversion clock source

• Interrupt control

9.1.1

PORT CONFIGURATION

When converting analog signals, the I/O pin selected

as the input channel should be configured for analog by

setting the associated TRIS and ANSEL bits. Refer to

Section 6.0 “I/O Ports” for more information.

9.1.2

CHANNEL SELECTION

There are 14 channel selections available:

-AN<11:0> pins

- Temperature Indicator

- FVR (Fixed Voltage Reference) Output

Refer to Section 11.0 “Temperature Indicator Mod-

ule” and Section 10.0 “Fixed Voltage Reference” for

more information on these channel selections.

The CHS bits of the ADCON0 register determine which

channel is connected to the sample and hold circuit.

When changing channels, a delay is required before

starting the next conversion. Refer to Section 9.2

“ADC Operation” for more information.

9.1.3

CONVERSION CLOCK

The source of the conversion clock is software

selectable via the ADCS bits of the ADCON1 register.

There are seven possible clock options:

•FOSC/2

•FOSC/4

•FOSC/8

•FOSC/16

•FOSC/32

•FOSC/64

•FRC (dedicated internal oscillator)

The time to complete one bit conversion is defined as

TAD. One full 8-bit conversion requires 10 TAD periods

as shown in Figure 9-2.

For

correct

conversion,

the

appropriate

TAD

specification must be met. Refer to the A/D conversion

requirements

Section 23.0

“Electrical

Specifications” for more information. Table 9-1 gives

examples of appropriate ADC clock selections.

Note:

Analog voltages on any pin that is defined

as a digital input may cause the input buf-

fer to conduct excess current.

Note:

Unless using the FRC, any changes in the

system clock frequency will change the

ADC

clock

frequency,

which

may

adversely affect the ADC result.

TABLE 9-1:

ADC CLOCK PERIOD (TAD) VS. DEVICE OPERATING FREQUENCIES

ADC Clock Period (TAD)

Device Frequency (FOSC)

ADC

Clock Source

ADCS<2:0>

16 MHz

8 MHz

4 MHz

1 MHz

FOSC/2

000

125 ns(2)

250 ns(2)

500 ns(2)

2.0

s

FOSC/4

100

250 ns(2)

500 ns(2)

1.0

s4.0 s

FOSC/8

001

0.5

s(2)

1.0

s2.0 s8 s(5)

FOSC/16

101

1.0

s2.0 s4.0 s16.0 s(5)

FOSC/32

010

2.0

s4.0 s8 s(5)

32.0

s(3)

FOSC/64

110

4.0

s8 s(5)

16.0

s(5)

64.0

s(3)

FRC

x11

1.0-6.0

s(1,4)

1.0-6.0

s(1,4)

1.0-6.0

s(1,4)

1.0-6.0

s(1,4)

Legend: Shaded cells are outside of recommended range.

Note 1:

The FRC source has a typical TAD time of 1.6

s for VDD.

These values violate the minimum required TAD time.

For faster conversion times, the selection of another clock source is recommended.

When the device frequency is greater than 1 MHz, the FRC clock source is only recommended if the

conversion will be performed during Sleep.

Recommended values for VDD

 2.0V and temperature -40°C to 85°C. The 16.0 s setting should be

avoided for temperature > 85°C.

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