PIC16F870SLASHP datasheet(84/156 Pages) MICROCHIP

84 / 156 page

PIC16F870/871

DS30569A-page 84

Preliminary

10.2

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 RC oscillator

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 10-1shows the resultant TAD times derived from

the device operating frequencies and the A/D clock

source selected.

TABLE 10-1:

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

10.3

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 recommended for sleep

operation.

3: For extended voltage devices (LC), please refer to the Electrical Specifications section.

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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