PIC876 datasheet(117/218 Pages) MICROCHIP

117 / 218 page

 2001 Microchip Technology Inc.

DS30292C-page 115

PIC16F87X

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 12TAD per 10-bit

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

software selected. The four possible options for TAD

are:

• 2TOSC

• 8TOSC

• 32TOSC

• 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.

TABLE 11-1:

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

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.

AD Clock Source (TAD)

Maximum Device Frequency

Operation

ADCS1:ADCS0

Max.

2TOSC

1.25 MHz

8TOSC

5 MHz

32TOSC

20 MHz

RC(1, 2, 3)

(Note 1)

Note 1: The RC source has a typical TAD time of 4

µs, but can vary between 2-6 µs.

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

mended for SLEEP operation.

3: For extended voltage devices (LC), please refer to the Electrical Characteristics (Sections 15.1 and 15.2).

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 ana-

log input. Analog levels on a digitally

configured input will not affect the conver-

sion 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.

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