PIC16F818-ESLASHML datasheet(87/176 Pages) MICROCHIP

87 / 176 page

 2003 Microchip Technology Inc.

DS39598D-page 85

PIC16F818/819

11.2

Selecting the A/D Conversion

Clock

The A/D conversion time per bit is defined as TAD. The

A/D conversion requires 9.0 TAD per 10-bit conversion.

The source of the A/D conversion clock is software

selectable. The seven possible options for TAD are:

•2 TOSC

•4 TOSC

•8 TOSC

•16 TOSC

•32 TOSC

•64 TOSC

• Internal A/D module RC oscillator (2-6

µs)

For correct A/D conversions, the A/D conversion clock

(TAD) must be selected to ensure a minimum TAD time

as small as possible, but no less than 1.6

µs and not

greater than 6.4

µs.

Table 11-1 shows the resultant TAD times derived from

the device operating frequencies and the A/D clock

source selected.

11.3

Configuring Analog Port Pins

The ADCON1 and TRISA registers control the opera-

tion of the A/D port pins. The port pins that are desired

as analog inputs must have their corresponding TRIS

bits set (input). If the TRIS bit is cleared (output), the

digital output level (VOH or VOL) will be converted.

The A/D operation is independent of the state of the

CHS<2:0> bits and the TRIS bits.

TABLE 11-1:

TAD vs. MAXIMUM DEVICE OPERATING FREQUENCIES (STANDARD DEVICES (F))

Note 1: When reading the Port register, all pins

configured as analog input channels will

read as cleared (a low level). Pins config-

ured as digital inputs will convert an

analog input. Analog levels on a digitally

configured

input

will

not

affect

the

conversion accuracy.

2: Analog levels on any pin that is defined as

a digital input (including the AN4:AN0

pins) may cause the input buffer to

consume current out of the device

specification.

AD Clock Source (TAD)

Maximum Device Frequency

Operation

ADCS<2>

ADCS<1:0>

2 TOSC

000

1.25 MHz

4 TOSC

100

2.5 MHz

8 TOSC

001

5 MHz

16 TOSC

101

10 MHz

32 TOSC

010

20 MHz

64 TOSC

110

20 MHz

RC(1,2,3)

X11

(Note 1)

Note 1:

The RC source has a typical TAD time of 4

µs but can vary between 2-6 µs.

When the device frequencies are greater than 1 MHz, the RC A/D conversion clock source is only

recommended for Sleep operation.

For extended voltage devices (LF), please refer to Section 15.0 “Electrical Characteristics”.

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