PIC18F87J10 Datasheet, PDF (255/394 Pages) Microchip Technology – 64/80-Pin High-Performance, 1-Mbit Flash Microcontrollers with nanoWatt Technology

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20.2 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 ACQT2:ACQT0
bits (ADCON2<5:3>) remain in their Reset state (‘000’)
and is compatible with devices that do not offer
programmable acquisition times.
If desired, the ACQT 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.
20.3 Selecting the A/D Conversion
Clock
The A/D conversion time per bit is defined as TAD. The
A/D conversion requires 11 TAD per 10-bit conversion.
The source of the A/D conversion clock is software
selectable.
There are seven possible options for TAD:
• 2 TOSC
• 4 TOSC
• 8 TOSC
• 16 TOSC
• 32 TOSC
• 64 TOSC
• 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 (approximately 2 µs, see parameter 130
for more information).
Table 20-1 shows the resultant TAD times derived from
the device operating frequencies and the A/D clock
source selected.
TABLE 20-1: TAD vs. DEVICE OPERATING
FREQUENCIES
AD Clock Source (TAD)
Operation ADCS2:ADCS0
Maximum
Device
Frequency
2 TOSC
000
1.25 MHz
4 TOSC
100
2.50 MHz
8 TOSC
001
5.00 MHz
16 TOSC
101
10.0 MHz
32 TOSC
010
20.0 MHz
64 TOSC
RC⁽³⁾
110
40.0 MHz
x11
1.00 MHz⁽¹⁾
Note 1: The RC source has a typical TAD time of
4 ms.
2: The RC source has a typical TAD time of
6 ms.
3: 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.
20.4 Configuring Analog Port Pins
The ADCON1, TRISA, TRISF and TRISH registers
control the operation of the A/D port pins. The port pins
needed as analog inputs must have their correspond-
ing TRIS bits set (input). If the TRIS 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
CHS3:CHS0 bits and the TRIS 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.
 2005 Microchip Technology Inc.
Advance Information
DS39663A-page 253