PIC18F258-ESLASHLSQTP datasheet(248/402 Pages) MICROCHIP

248 / 402 page

PIC18FXX8

DS41159E-page 246

20.2

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 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 20-1 shows the resultant TAD times derived from

the device operating frequencies and the A/D clock

source selected.

20.3

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 20-1:

TAD vs. DEVICE OPERATING FREQUENCIES

TABLE 20-2:

TAD vs. DEVICE OPERATING FREQUENCIES (FOR EXTENDED, LF DEVICES)

Note 1: When reading the port register, all pins

configured as analog input channels will

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

ured 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

consume current that is out of the

device’s specification.

AD Clock Source (TAD)

Device Frequency

Operation

ADCS2:ADCS0

20 MHz

5 MHz

1.25 MHz

333.33 kHz

2 TOSC

000

100 ns(2)

400 ns(2)

1.6

μs6 μs

4 TOSC

100

200 ns(2)

800 ns(2)

3.2

μs12 μs

8 TOSC

001

400 ns(2)

1.6

μs6.4 μs

μs(3)

16 TOSC

101

800 ns(2)

3.2

μs

12.8

μs

μs(3)

32 TOSC

010

1.6

μs6.4 μs

25.6

μs(3)

64 TOSC

110

3.2

μs12.8 μs

51.2

μs(3)

192

μs(3)

011

2-6

μs(1)

2-6

μs(1)

2-6

μs(1)

2-6

μs(1)

Legend: Shaded cells are outside of recommended range.

Note 1:

The RC source has a typical TAD time of 4

μs.

These values violate the minimum required TAD time.

For faster conversion times, the selection of another clock source is recommended.

AD Clock Source (TAD)

Device Frequency

Operation

ADCS2:ADCS0

4 MHz

2 MHz

1.25 MHz

333.33 kHz

2 TOSC

000

500 ns(2)

1.0

μs(2)

1.6

μs(2)

μs

4 TOSC

100

1.0

μs(2)

2.0

μs(2)

3.2

μs(2)

μs

8 TOSC

001

2.0

μs(2)

4.0

μs6.4 μs

μs(3)

16 TOSC

101

4.0

μs(2)

8.0

μs

12.8

μs

μs(3)

32 TOSC

010

8.0

μs16.0 μs

25.6

μs(3)

64 TOSC

110

16.0

μs32.0 μs

51.2

μs(3)

192

μs(3)

011

3-9

μs(1)

3-9

μs(1)

3-9

μs(1)

3-9

μs(1)

Legend: Shaded cells are outside of recommended range.

Note 1:

The RC source has a typical TAD time of 6

μs.

These values violate the minimum required TAD time.

For faster conversion times, the selection of another clock source is recommended.

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