PIC18F4520ESLASHMLQTP datasheet(233/390 Pages) MICROCHIP

233 / 390 page

 2004 Microchip Technology Inc.

Preliminary

DS39631A-page 231

PIC18F2420/2520/4420/4520

19.6

A/D Conversions

Figure 19-4 shows the operation of the A/D converter

after the GO bit has been set and the ACQT2:ACQT0

bits are cleared. A conversion is started after the follow-

ing instruction to allow entry into Sleep mode before the

conversion begins.

Figure 19-5 shows the operation of the A/D converter

after the GO bit has been set and the ACQT2:ACQT0

bits are set to ‘010’ and selecting a 4 TAD acquisition

time before the conversion starts.

Clearing the GO/DONE bit during a conversion will abort

the current conversion. The A/D Result register pair will

NOT be updated with the partially completed A/D

conversion sample. This means the ADRESH:ADRESL

registers will continue to contain the value of the last

completed conversion (or the last value written to the

ADRESH:ADRESL registers).

After the A/D conversion is completed or aborted, a

2TAD wait is required before the next acquisition can

be started. After this wait, acquisition on the selected

channel is automatically started.

19.7

Discharge

The discharge phase is used to initialize the value of

the capacitor array. The array is discharged before

every sample. This feature helps to optimize the unity-

gain amplifier, as the circuit always needs to charge the

capacitor array, rather than charge/discharge based on

previous measure values.

FIGURE 19-4:

A/D CONVERSION TAD CYCLES (ACQT<2:0> = 000, TACQ = 0)

FIGURE 19-5:

A/D CONVERSION TAD CYCLES (ACQT<2:0> = 010, TACQ = 4 TAD)

Note:

The GO/DONE bit should NOT be set in

the same instruction that turns on the A/D.

TAD1 TAD2 TAD3 TAD4 TAD5 TAD6 TAD7 TAD8

TAD11

Set GO bit

Holding capacitor is disconnected from analog input (typically 100 ns)

TAD9 TAD10

TCY - TAD

ADRESH:ADRESL is loaded, GO bit is cleared,

ADIF bit is set, holding capacitor is connected to analog input.

Conversion starts

On the following cycle:

TAD1

Discharge

Set GO bit

(Holding capacitor is disconnected)

Conversion starts

(Holding capacitor continues

acquiring input)

TACQT Cycles

TAD Cycles

Automatic

Acquisition

Time

ADRESH:ADRESL is loaded, GO bit is cleared,

ADIF bit is set, holding capacitor is connected to analog input.

On the following cycle:

TAD1

Discharge

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