PIC18FXX20
19.4 A/D Conversions
19.5 Use of the CCP2 Trigger
Figure 19-3 shows the operation of the A/D converter
after the GO bit has been set. Clearing the GO/DONE
bit during a conversion will abort the current conver-
sion. The A/D result register pair will NOT be updated
with the partially completed A/D conversion sample.
That is, the ADRESH:ADRESL registers will continue
to contain the value of the last completed conversion
(or the last value written to the ADRESH:ADRESL reg-
isters). After the A/D conversion is aborted, a 2 TAD wait
is required before the next acquisition is started. After
this 2 TAD wait, acquisition on the selected channel is
automatically started.
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 1011and 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).
Note: The GO/DONE bit should NOT be set in
the same instruction that turns on the A/D.
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
TCY - TAD
TAD7 TAD8 TAD9 TAD10 TAD11
TAD1 TAD2 TAD3 TAD4 TAD5 TAD6
b0
b5
b4
b3
b2
b1
b0
b7
b6
b8
b9
Conversion Starts
Holding capacitor is disconnected from analog input (typically 100 ns)
Set GO bit
Next Q4: ADRESH/ADRESL is loaded, GO bit is cleared,
ADIF bit is set, holding capacitor is connected to analog input.
DS39609A-page 220
Advance Information
2003 Microchip Technology Inc.
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