PIC16LC773-04SLASHSP datasheet(121/200 Pages) MICROCHIP

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PIC16C77X

Advance Information

DS30275A-page 121

11.4

Selecting the A/D Conversion Clock

The A/D conversion cycle requires 13TAD: 1 TAD for set-

tling time, and 12 TAD for conversion. The source of the

A/D conversion clock is software selected. The four

possible options for TAD are:

•2 TOSC

•8 TOSC

•32 TOSC

• Internal RC oscillator

Note that these options are the same as those of the

8-bit A/D.

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 11-1 shows the resultant TAD times

derived from the device operating frequencies and the

A/D clock source selected.

The ADIF bit is set on the rising edge of the 14th TAD.

The GO/DONE bit is cleared on the falling edge of the

14th TAD.

TABLE 11-1

TAD vs. DEVICE OPERATING FREQUENCIES

11.5

A/D Conversions

Figure 11-5 shows an example that performs an A/D

conversion. The port pins are configured as analog

inputs. The analog reference VREF+ is the device AVDD

and the analog reference VREF- is the device AVSS. The

A/D interrupt is enabled, and the A/D conversion clock

is TRC. The conversion is performed on the AN0 chan-

nel.

FIGURE 11-4: PERFORMING AN A/D CONVERSION

BCF

PIR1, ADIF

;Clear A/D Int Flag

BSF

STATUS, RP0

;Select Page 1

CLRF

ADCON1

;Configure A/D Inputs

BSF

PIE1, ADIE

;Enable A/D interrupt

BCF

STATUS, RP0

;Select Page 0

MOVLW

0xC1

;RC clock, A/D is on,

;Ch 0 is selected

MOVWF

ADCON0

;

BSF

INTCON, PEIE

;Enable Peripheral

BSF

INTCON, GIE

;Enable All Interrupts

;

; Ensure that the required sampling time for the

; selected input channel has lapsed. Then the

; conversion may be started.

BSF

ADCON0, GO

;Start A/D Conversion

;The ADIF bit will be

;set and the GO/DONE bit

;cleared upon completion-

;of the A/D conversion.

AD Clock Source (TAD)

Device Frequency

Operation

ADCS<1:0>

20 MHz

5 MHz

4 MHz

1.25 MHz

2 TOSC

100 ns(2)

400 ns(2)

500 ns(2)

1.6

µs

8 TOSC

800 ns(2)

1.6

µs2.0 µs6.4 µs

32 TOSC

1.6

µs6.4 µs

8.0

µs(3)

2 - 6

µs(1,4)

2 - 6

µs(1,4)

2 - 6

µs(1,4)

2 - 6

µs(1,4)

Note 1: The RC source has a typical TAD time of 4

µs for VDD > 3.0V.

2: These values violate the minimum required TAD time.

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

4: When the device frequency is greater than 1 MHz, the RC A/D conversion clock source is only recommended if the conver-

sion will be performed during sleep.

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