PIC18F242 datasheet(189/332 Pages) MICROCHIP

189 / 332 page

DS39564C-page 187

PIC18FXX2

17.4

A/D Conversions

Figure 17-3 shows the operation of the A/D converter

after the GO bit has been set. Clearing the GO/DONE

bit during a conversion will abort the current conver-

sion. The A/D result register pair will NOT be updated

with the partially completed A/D conversion sample.

That is, the ADRESH:ADRESL registers will continue

to contain the value of the last completed conversion

(or the last value written to the ADRESH:ADRESL reg-

isters). After the A/D conversion is aborted, a 2 TAD wait

is required before the next acquisition is started. After

this 2 TAD wait, acquisition on the selected channel is

automatically started. The GO/DONE bit can then be

set to start the conversion.

FIGURE 17-3:

A/D CONVERSION TAD CYCLES

17.4.1

A/D RESULT REGISTERS

The ADRESH:ADRESL register pair is the location

where the 10-bit A/D result is loaded at the completion

of the A/D conversion. This register pair is 16-bits wide.

The A/D module gives the flexibility to left or right justify

the 10-bit result in the 16-bit result register. The A/D

Format Select bit (ADFM) controls this justification.

Figure 17-4 shows the operation of the A/D result justi-

fication. The extra bits are loaded with ’0’s. When an

A/D result will not overwrite these locations (A/D

disable), these registers may be used as two general

purpose 8-bit registers.

FIGURE 17-4:

A/D RESULT JUSTIFICATION

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

Next Q4: ADRESH/ADRESL is loaded, GO bit is cleared,

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

Conversion Starts

10-bit Result

ADRESH

ADRESL

0000 00

ADFM = 0

2 1 0 7

10-bit Result

ADRESH

ADRESL

10-bit Result

0000 00

0 7 6 5

ADFM = 1

Right Justified

Left Justified

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