PIC18F1XK22 Datasheet, PDF (208/388 Pages) Microchip Technology – 20-Pin Flash Microcontrollers with nanoWatt XLP Technology

PIC18F1XK22/LF1XK22
16.1 ADC Configuration
When configuring and using the ADC the following
functions must be considered:
• Port configuration
• Channel selection
• ADC voltage reference selection
• ADC conversion clock source
• Interrupt control
• Results formatting
16.1.1 PORT CONFIGURATION
The ANSEL, ANSELH, TRISA, TRISB and TRISE reg-
isters all configure the A/D port pins. Any port pin
needed as an analog input should have its correspond-
ing ANSx bit set to disable the digital input buffer and
TRISx bit set to disable the digital output driver. If the
TRISx bit is cleared, the digital output level (VOH or
VOL) will be converted.
The A/D operation is independent of the state of the
ANSx bits and the TRIS bits.
Note 1: When reading the PORT register, all pins
with their corresponding ANSx bit set
read as cleared (a low level). However,
analog conversion of pins configured as
digital inputs (ANSx bit cleared and
TRISx bit set) will be accurately
converted.
2: Analog levels on any pin with the corre-
sponding ANSx bit cleared may cause the
digital input buffer to consume current out
of the device’s specification limits.
16.1.2 CHANNEL SELECTION
The CHS bits of the ADCON0 register determine which
channel is connected to the sample and hold circuit.
When changing channels, a delay is required before
starting the next conversion. Refer to Section 16.2
“ADC Operation” for more information.
16.1.3 ADC VOLTAGE REFERENCE
The PVCFG and NVCFG bits of the ADCON1 register
provide independent control of the positive and
negative voltage references, respectively. The positive
voltage reference can be either VDD, FVR or an
external voltage source. The negative voltage
reference can be either VSS or an external voltage
source.
16.1.4 SELECTING AND CONFIGURING
ACQUISITION TIME
The ADCON2 register allows the user to select an
acquisition time that occurs each time the GO/DONE
bit is set.
Acquisition time is set with the ACQT<2:0> bits of the
ADCON2 register. Acquisition delays cover a range of
2 to 20 TAD. When the GO/DONE bit is set, the A/D
module continues to sample the input for the selected
acquisition time, then automatically begins a conver-
sion. Since the acquisition time is programmed, there is
no need to wait for an acquisition time between select-
ing a channel and setting the GO/DONE bit.
Manual acquisition is selected when
ACQT<2:0> = 000. When the GO/DONE bit is set,
sampling is stopped and a conversion begins. The user
is responsible for ensuring the required acquisition time
has passed between selecting the desired input
channel and setting the GO/DONE bit. This option is
also the default Reset state of the ACQT<2:0> bits and
is compatible with devices that do not offer
programmable acquisition times.
In either case, when the conversion is completed, the
GO/DONE bit is cleared, the ADIF flag is set and the
A/D begins sampling the currently selected channel
again. When an acquisition time is programmed, there
is no indication of when the acquisition time ends and
the conversion begins.
16.1.5 CONVERSION CLOCK
The source of the conversion clock is software select-
able via the ADCS bits of the ADCON2 register. There
are seven possible clock options:
• FOSC/2
• FOSC/4
• FOSC/8
• FOSC/16
• FOSC/32
• FOSC/64
• FRC (dedicated internal oscillator)
The time to complete one bit conversion is defined as
TAD. One full 10-bit conversion requires 11 TAD periods
as shown in Figure 16-3.
For correct conversion, the appropriate TAD specification
must be met. See A/D conversion requirements in
Table 25-9 for more information. Table 16-1 gives
examples of appropriate ADC clock selections.
Note:
Unless using the FRC, any changes in the
system clock frequency will change the
ADC clock frequency, which may
adversely affect the ADC result.
DS41365D-page 208
Preliminary
 2010 Microchip Technology Inc.