Datasheet

© 2006 Microchip Technology Inc. DS39564C-page 183
PIC18FXX2
The analog reference voltage is software selectable to
either the device’s positive and negative supply voltage
(V
DD and VSS), or the voltage level on the RA3/AN3/
V
REF+ pin and RA2/AN2/VREF- pin.
The A/D converter has a unique feature of being able
to operate while the device is in SLEEP mode. To oper-
ate in SLEEP, the A/D conversion clock must be
derived from the A/D’s internal RC oscillator.
The output of the sample and hold is the input into the
converter, which generates the result via successive
approximation.
A device RESET forces all registers to their RESET
state. This forces the A/D module to be turned off and
any conversion is aborted.
Each port pin associated with the A/D converter can be
configured as an analog input (RA3 can also be a
voltage reference) or as a digital I/O.
The ADRESH and ADRESL registers contain the result
of the A/D conversion. When the A/D conversion is
complete, the result is loaded into the ADRESH/
ADRESL registers, the GO/DONE
bit (ADCON0<2>) is
cleared, and A/D interrupt flag bit, ADIF is set. The block
diagram of the A/D module is shown in Figure 17-1.
FIGURE 17-1: A/D BLOCK DIAGRAM
(Input Voltage)
VAIN
VREF+
Reference
Voltage
V
DD
PCFG<3:0>
CHS<2:0>
AN7*
AN6*
AN5*
AN4
AN3
AN2
AN1
AN0
111
110
101
100
011
010
001
000
10-bit
Converter
VREF-
VSS
A/D
* These channels are implemented only on the PIC18F4X2 devices.