PIC18F242 datasheet(188/332 Pages) MICROCHIP

188 / 332 page

PIC18FXX2

DS39564C-page 186

17.2

Selecting the A/D Conversion Clock

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

A/D conversion requires 12 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

of 1.6

μs.

Table 17-1 shows the resultant TAD times derived from

the device operating frequencies and the A/D clock

source selected.

17.3

Configuring Analog Port Pins

The ADCON1, TRISA and TRISE 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 17-1:

TAD vs. DEVICE OPERATING FREQUENCIES

Note 1: When reading the port register, all pins con-

figured as analog input channels will read

as cleared (a low level). Pins configured as

digital inputs will convert an analog 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 AN4:AN0

pins) may cause the input buffer to con-

sume current that is out of the device’s

specification.

AD Clock Source (TAD)

Maximum Device Frequency

Operation

ADCS2:ADCS0

PIC18FXX2

PIC18LFXX2

2 TOSC

000

1.25 MHz

666 kHz

4 TOSC

100

2.50 MHz

1.33 MHz

8 TOSC

001

5.00 MHz

2.67 MHz

16 TOSC

101

10.00 MHz

5.33 MHz

32 TOSC

010

20.00 MHz

10.67 MHz

64 TOSC

110

40.00 MHz

21.33 MHz

011

——

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