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.
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
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.
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.
The A/D conversion time per bit is defined as T
AD
. The
A/D conversion requires a minimum 12 T
AD
per 10-bit
conversion. The source of the A/D conversion clock is
software selected. The seven possible options for T
AD
are:
•
•
•
•
•
•
•
2 T
OSC
4 T
OSC
8 T
OSC
16 T
OSC
32 T
OSC
64 T
OSC
Internal A/D module, RC oscillator (2-6
µs)
For correct A/D conversions, the A/D conversion clock
(T
AD
) must be selected to ensure a minimum T
AD
time
of 1.6
µs.
AD
times derived from
the device operating frequencies and the A/D clock
source selected.
TABLE 12-1:
T
AD
vs. MAXIMUM DEVICE OPERATING FREQUENCIES (STANDARD DEVICES (F))
AD Clock Source (T
AD
)
ADCS2:ADCS1:ADCS0
000
100
001
101
010
110
x11
Maximum Device Frequency
1.25 MHz
2.5 MHz
5 MHz
10 MHz
20 MHz
20 MHz
(Note 1)
Operation
2 T
OSC
4 T
OSC
8 T
OSC
16 T
OSC
32 T
OSC
64 T
OSC
RC
(1,2,3)
Note 1:
2:
3:
The RC source has a typical T
AD
time of 4
µs
but can vary between 2-6
µs.
When the device frequencies are greater than 1 MHz, the RC A/D conversion clock source is only
recommended for Sleep operation.
For extended voltage devices (LF), please refer to
2004 Microchip Technology Inc.
DS30498C-page 157
MICROCHIP [ MICROCHIP TECHNOLOGY ]
