PIC16F7X7
12.2 Selecting and Configuring
Automatic Acquisition Time
12.3 Selecting the A/D Conversion
Clock
The ADCON2 register allows the user to select an
acquisition time that occurs each time the GO/DONE
bit is set.
The A/D conversion time per bit is defined as TAD. The
A/D conversion requires a minimum 12 TAD per 10-bit
conversion. The source of the A/D conversion clock is
software selected. The seven possible options for TAD
are:
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.
• 2 TOSC
• 4 TOSC
• 8 TOSC
• 16 TOSC
• 32 TOSC
• 64 TOSC
If desired, the ACQT bits can be set to select a
programmable acquisition time for the A/D module.
When the GO/DONE bit is set, the A/D module con-
tinues to sample the input for the selected acquisition
time, then automatically begins a conversion. Since the
acquisition time is programmed, there may be no need
to wait for an acquisition time between selecting a
channel and setting the GO/DONE bit.
• Internal A/D module, RC oscillator (2-6 µs)
For correct A/D conversions, the A/D conversion clock
(TAD) must be selected to ensure a minimum TAD time
of 1.6 µs.
Table 12-1 shows the resultant TAD times derived from
the device operating frequencies and the A/D clock
source selected.
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.
TABLE 12-1: TAD vs. MAXIMUM DEVICE OPERATING FREQUENCIES (STANDARD DEVICES (F))
AD Clock Source (TAD)
Maximum Device Frequency
ADCS2:ADCS1:ADCS0
Operation
2 TOSC
4 TOSC
000
100
001
101
010
110
x11
1.25 MHz
2.5 MHz
5 MHz
8 TOSC
16 TOSC
32 TOSC
64 TOSC
RC(1,2,3)
10 MHz
20 MHz
20 MHz
(Note 1)
Note 1: The RC source has a typical TAD time of 4 µs but can vary between 2-6 µs.
2: When the device frequencies are greater than 1 MHz, the RC A/D conversion clock source is only
recommended for Sleep operation.
3: For extended voltage devices (LF), please refer to Section 18.0 “Electrical Characteristics”.
2004 Microchip Technology Inc.
DS30498C-page 157
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