16F87 datasheet(120/228 Pages) MICROCHIP

120 / 228 page

PIC16F87/88

DS30487C-page 118

 2005 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 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 12-1 shows the resultant TAD times derived from

the device operating frequencies and the A/D clock

source selected.

12.3

Operation in Power-Managed

Modes

The selection of the automatic acquisition time and

A/D conversion clock is determined in part by the clock

source and frequency while in a power-managed

mode.

If the A/D is expected to operate while the device is in

a power-managed mode, the ADCS2:ADCS0 bits in

ADCON0 and ADCON1 should be updated in accor-

dance with the power-managed mode clock that will be

used. After the power-managed mode is entered

(either of the power-managed Run modes), an A/D

acquisition or conversion may be started. Once an

acquisition or conversion is started, the device should

continue to be clocked by the same power-managed

mode clock source until the conversion has been

completed.

If the power-managed mode clock frequency is less

than 1 MHz, the A/D RC clock source should be

selected.

TABLE 12-1:

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

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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