DM163006 datasheet(199/320 Pages) MICROCHIP

199 / 320 page

 2001 Microchip Technology Inc.

Advance Information

DS39541A-page 199

PIC18C601/801

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. There are seven possible options for TAD:

• 2TOSC

• 4TOSC

• 8TOSC

• 16TOSC

• 32TOSC

• 64TOSC

• 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 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, TRISF and TRISH registers

control the operation of the A/D port pins. The port pins

needed as analog inputs must have their correspond-

ing TRIS bits set (input). If the TRIS bit is cleared (out-

put), the digital output level (VOH or VOL) will be

converted.

The A/D operation is independent of the state of the

CHS3:CHS0 bits and the TRIS bits.

TABLE 17-1:

TAD vs. DEVICE OPERATING FREQUENCIES

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 ana-

log input. Analog levels on a digitally

configured input will not affect the conver-

sion accuracy.

2: Analog levels on any pin defined as a dig-

ital input may cause the input buffer to

consume current out of the device’s spec-

ification limits.

AD Clock Source (TAD)

Maximum Device Frequency

Operation

ADCS2:ADCS0

PIC18C601/801

PIC18LC601/801(5)

2TOSC

000

1.25 MHz

666 kHz

4TOSC

100

2.50 MHz

1.33 MHz

8TOSC

001

5.00 MHz

2.67 MHz

16TOSC

101

10.0 MHz

5.33 MHz

32TOSC

010

20.0 MHz

10.67 MHz

64TOSC

110

——

x11

——

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

4 µs.

2: These values violate the minimum required TAD time.

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

4: For device frequencies above 1 MHz, the device must be in SLEEP for the entire conversion or the A/D

accuracy may be out of specification.

5: This column is for the LC devices only.

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