PIC876 datasheet(118/218 Pages) MICROCHIP

118 / 218 page

PIC16F87X

DS30292C-page 116

 2001 Microchip Technology Inc.

11.4

A/D Conversions

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. 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 registers). After the A/D conversion

is aborted, a 2TAD wait is required before the next

acquisition is started. After this 2TAD wait, acquisition

on the selected channel is automatically started. The

GO/DONE bit can then be set to start the conversion.

In Figure 11-3, after the GO bit is set, the first time seg-

ment has a minimum of TCY and a maximum of TAD.

FIGURE 11-3:

A/D CONVERSION TAD CYCLES

11.4.1

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 11-4 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 dis-

able), these registers may be used as two general

purpose 8-bit registers.

FIGURE 11-4:

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

TAD9

Set GO bit

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

TAD10 TAD11

TCY to TAD

Conversion starts

ADRES is loaded

GO bit is cleared

ADIF bit is set

Holding capacitor is connected to analog input

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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