PIC18F87K22 Datasheet, PDF (360/548 Pages) Microchip Technology – 64/80-Pin, High-Performance, 1-Mbit Enhanced Flash Microcontrollers with 12-Bit A/D and nanoWatt XLP Technology

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23.4 Selecting and Configuring
Automatic Acquisition Time
The ADCON2 register allows the user to select an
acquisition time that occurs each time the GO/DONE
bit is set.
When the GO/DONE bit is set, sampling is stopped and
a conversion begins. The user is responsible for ensur-
ing the required acquisition time has passed between
selecting the desired input channel and setting the
GO/DONE bit.
This occurs when the ACQT<2:0> bits
(ADCON2<5:3>) remain in their Reset state (‘000’),
which is compatible with devices that do not offer
programmable acquisition times.
If desired, the ACQTx bits can be set to select a pro-
grammable acquisition time for the A/D module. 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 conversion. Since the acquisi-
tion time is programmed, there may be no need to wait
for an acquisition time between selecting a channel and
setting the GO/DONE bit.
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. If an acquisition time is programmed, there is
nothing to indicate if the acquisition time has ended or
if the conversion has begun.
23.5 Selecting the A/D Conversion
Clock
The A/D conversion time per bit is defined as TAD. The
A/D conversion requires 14 TAD per 12-bit conversion.
The source of the A/D conversion clock is
software-selectable.
The possible options for TAD are:
• 2 TOSC
• 4 TOSC
• 8 TOSC
• 16 TOSC
• 32 TOSC
• 64 TOSC
• Using the internal RC Oscillator
For correct A/D conversions, the A/D conversion clock
(TAD) must be as short as possible but greater than the
minimum TAD. (For more information, see parameter
130 in Table 31-27.)
Table 23-1 shows the resultant TAD times derived from
the device operating frequencies and the A/D clock
source selected.
TABLE 23-1: TAD vs. DEVICE OPERATING
FREQUENCIES
AD Clock Source (TAD)
Operation
ADCS<2:0>
Maximum
Device
Frequency
2 TOSC
000
2.86 MHz
4 TOSC
100
5.71 MHz
8 TOSC
001
11.43 MHz
16 TOSC
101
22.86 MHz
32 TOSC
010
40.00 MHz
64 TOSC
RC⁽²⁾
110
40.00 MHz
x11
1.00 MHz⁽¹⁾
Note 1: The RC source has a typical TAD time of
4 s.
2: For device frequencies above 1 MHz, the
device must be in Sleep mode for the
entire conversion or the A/D accuracy may
be out of specification.
23.6 Configuring Analog Port Pins
The ANCON0, ANCON1, ANCON2, TRISA, TRISF,
TRISG and TRISH registers control the operation of the
A/D port pins. The port pins needed as analog inputs
must have their corresponding TRISx bits set (input). If
the TRISx bit is cleared (output), the digital output level
(VOH or VOL) will be converted.
The A/D operation is independent of the state of the
CHS<3:0> bits and the TRISx bits.
Note 1: When reading the PORT register, all pins
configured as analog input channels will
read as cleared (a low level). Pins config-
ured as digital inputs will convert an
analog input. Analog levels on a digitally
configured input will be accurately
converted.
2: Analog levels on any pin defined as a
digital input may cause the digital input
buffer to consume current out of the
device’s specification limits.
DS39960B-page 360
Preliminary
 2010 Microchip Technology Inc.