Datasheet

PIC18F6585/8585/6680/8680
DS30491D-page 256  2003-2013 Microchip Technology Inc.
19.6 A/D Conversions
Figure 19-3 shows the operation of the A/D converter
after the GO bit has been set and the ACQT2:ACQT0
bits are cleared. A conversion is started after the follow-
ing instruction to allow entry into Sleep mode before the
conversion begins.
Figure 19-4 shows the operation of the A/D converter
after the GO bit has been set, the ACQT2:ACQT0 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.
19.7 Use of the CCP2 Trigger
An A/D conversion can be started by the “special event
trigger” of the CCP2 module. This requires that the
CCP2M3:CCP2M0 bits (CCP2CON<3:0>) be pro-
grammed as ‘1011’ and that the A/D module is enabled
(ADON bit is set). When the trigger occurs, the
GO/DONE
bit will be set, starting the A/D conversion
and the Timer1 (or Timer3) counter will be reset to zero.
Timer1 (or Timer3) is reset to automatically repeat the
A/D acquisition period with minimal software overhead
(moving ADRESH/ADRESL to the desired location).
The appropriate analog input channel must be selected
and the minimum acquisition done before the “special
event trigger” sets the GO/DONE
bit (starts a
conversion).
If the A/D module is not enabled (ADON is cleared), the
“special event trigger” will be ignored by the A/D
module but will still reset the Timer1 (or Timer3)
counter.
FIGURE 19-3: A/D CONVERSION TAD CYCLES (ACQT<2:0> = 000, TACQ = 0)
FIGURE 19-4: 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 bit
Holding capacitor is disconnected from analog input (typically 100 ns)
TAD9 TAD10TCY - TAD
Next Q4: ADRESH/ADRESL is loaded, GO 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
1
2
3
4 5
6 7
8 11
Set GO bit
(Holding capacitor is disconnected)
9 10
Next Q4: ADRESH:ADRESL is loaded, GO bit is cleared,
ADIF bit is set, holding capacitor is reconnected to analog input.
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
18F8680.book Page 256 Tuesday, January 29, 2013 1:32 PM