Datasheet

Table Of Contents
© 2009 Microchip Technology Inc. DS39689F-page 241
PIC18F2221/2321/4221/4321 FAMILY
20.6 A/D Conversions
Figure 20-4 shows the operation of the A/D converter
after the GO/DONE
bit has been set and the
ACQT<2:0> bits are cleared. A conversion is started
after the following instruction to allow entry into Sleep
mode before the conversion begins.
Figure 20-5 shows the operation of the A/D converter
after the GO/DONE
bit has been set and the
ACQT<2:0> bits are set to ‘010’ and selecting a 4 T
AD
acquisition time before the conversion starts.
Clearing the GO/DONE
bit during a conversion will abort
the current conversion. The A/D Result register pair will
NOT be updated with the partially completed A/D
conversion sample. This means the ADRESH:ADRESL
registers will continue to contain the value of the last
completed conversion (or the last value written to the
ADRESH:ADRESL registers).
After the A/D conversion is completed or aborted, a
2T
AD wait is required before the next acquisition can be
started. After this wait, acquisition on the selected
channel is automatically started.
20.7 Discharge
The discharge phase is used to initialize the value of
the capacitor array. The array is discharged before
every sample. This feature helps to optimize the unity-
gain amplifier, as the circuit always needs to charge the
capacitor array, rather than charge/discharge based on
previous measure values.
FIGURE 20-4: A/D CONVERSION TAD CYCLES (ACQT<2:0> = 000, TACQ = 0)
FIGURE 20-5: A/D CONVERSION T
AD CYCLES (ACQT<2:0> = 010, TACQ = 4 TAD)
Note: The GO/DONE bit should NOT be set in
the same instruction that turns on the A/D.
TAD1
TAD2
TAD3
TAD4 TAD5
TAD6 TAD7
TAD8 TAD11
Set GO/DONE bit
Holding capacitor is disconnected from analog input (typically 100 ns)
TAD9 TAD10
TCY - TAD
ADRESH:ADRESL is loaded, GO/DONE bit is cleared,
ADIF bit is set, holding capacitor is connected to analog input.
Conversion starts
b0
b9
b6
b5
b4
b3
b2
b1
b8
b7
On the following cycle:
TAD1
Discharge
1
2
3 4 5
6
7
8
11
Set GO/DONE bit
(Holding capacitor is disconnected)
9
10
Conversion starts
1
2
3 4
(Holding capacitor continues
acquiring input)
T
ACQT Cycles
TAD Cycles
Automatic
Acquisition
Time
b0b9
b6
b5 b4
b3
b2
b1
b8
b7
ADRESH:ADRESL is loaded, GO/DONE
bit is cleared,
ADIF bit is set, holding capacitor is connected to analog input.
On the following cycle:
TAD1
Discharge