PIC16F877ATISLASHML datasheet(133/234 Pages) MICROCHIP

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 2003 Microchip Technology Inc.

DS39582B-page 131

PIC16F87XA

11.2

Selecting the A/D Conversion

Clock

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

A/D conversion requires a minimum 12 TAD per 10-bit

conversion. The source of the A/D conversion clock is

software selected. 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

of 1.6

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 TRIS registers control the operation

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

CHS2:CHS0 bits and the TRIS bits.

TABLE 11-1:

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

Note 1: When reading the port register, any pin

configured as an analog input channel will

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

ured as digital inputs will convert an analog

input. Analog levels on a digitally config-

ured input will not affect the conversion

accuracy.

2: Analog levels on any pin that is defined as

a digital input (including the AN7:AN0

pins) may cause the input buffer to con-

sume current that is out of the device

specifications.

AD Clock Source (TAD)

Maximum Device Frequency

Operation

ADCS2:ADCS1:ADCS0

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 17.0 “Electrical Characteristics”.

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