PIC16F87 datasheet(120/214 Pages) MICROCHIP

120 / 214 page

PIC16F87/88

DS30487B-page 118

Preliminary

 2003 Microchip Technology Inc.

12.2

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 8-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 12-1 shows the resultant TAD times derived from

the device operating frequencies and the A/D clock

source selected.

12.3

Configuring Analog Port Pins

The ADCON1, ANSEL, TRISA, and TRISB registers

control the operation of the A/D port pins. The port pins

that are desired as analog inputs must have their cor-

responding 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 12-1:

TAD vs. MAXIMUM DEVICE OPERATING FREQUENCIES – STANDARD DEVICES (C)

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 ana-

log 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 RA4:RA0 and

RB7:RB6 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>

Max.

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 18.0 “Electrical Characteristics”.

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