Datasheet
© 2009 Microchip Technology Inc. DS39755C-page 29
PIC18F2423/2523/4423/4523
The value in the ADRESH:ADRESL registers is
unknown following POR and BOR Resets and is not
affected by any other Reset.
After the A/D module has been configured as desired,
the selected channel must be acquired before the
conversion is started. The analog input channels must
have their corresponding TRIS bits selected as inputs.
To determine acquisition time, see Section 2.1 “A/D
Acquisition Requirements”.
After this acquisition time has elapsed, the A/D conver-
sion can be started. An acquisition time can be
programmed to occur between setting the GO/DONE
bit and the actual start of the conversion.
The following steps should be followed to perform an A/D
conversion:
1. Configure the A/D module:
• Configure analog pins, voltage reference and
digital I/O (ADCON1)
• Select A/D input channel (ADCON0)
• Select A/D acquisition time (ADCON2)
• Select A/D conversion clock (ADCON2)
• Turn on the A/D module (ADCON0)
2. Configure the A/D interrupt (if desired):
• Clear ADIF bit
• Set ADIE bit
• Set GIE bit
3. Wait the required acquisition time (if required).
4. Start conversion by setting the GO/DONE
bit
(ADCON0<1>).
5. Wait for the A/D conversion to complete by either:
• Polling for the GO/DONE
bit to be cleared
OR
• Waiting for the A/D interrupt
6. Read the A/D Result registers (ADRESH:ADRESL)
and clear the ADIF bit, if required.
7. For the next conversion, go to step 1 or step 2,
as required.
The A/D conversion time per bit is defined as
T
AD. A minimum wait of 2 TAD is required before
the next acquisition starts.
FIGURE 2-2: A/D TRANSFER FUNCTION
FIGURE 2-3: ANALOG INPUT MODEL
Digital Code Output
FFEh
003h
002h
001h
000h
0.5 LSB
1 LSB
1.5 LSB
2 LSB
2.5 LSB
4094 LSB
4094.5 LSB
3 LSB
Analog Input Voltage
FFFh
4095 LSB
4095.5 LSB
VAIN
CPIN
Rs
ANx
5 pF
V
T = 0.6V
V
T = 0.6V
I
LEAKAGE
RIC ≤ 1k
Sampling
Switch
SS
R
SS
CHOLD = 25 pF
V
SS
VDD
±100 nA
Legend: CPIN
VT
ILEAKAGE
RIC
SS
C
HOLD
= Input Capacitance
= Threshold Voltage
= Leakage Current at the pin due to
= Interconnect Resistance
= Sampling Switch
= Sample/Hold Capacitance (from DAC)
various junctions
= Sampling Switch ResistanceRSS
VDD
6V
Sampling Switch
5V
4V
3V
2V
1234
(kΩ)