PIC18F2420/2520/4420/4520
The value in the ADRESH:ADRESL registers is not
5. Wait for A/D conversion to complete, by either:
• Polling for the GO/DONE bit to be cleared
modified for a Power-on Reset. The ADRESH:ADRESL
registers will contain unknown data after a Power-on
Reset.
OR
• Waiting for the A/D interrupt
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 an
input. To determine acquisition time, see Section 19.1
“A/D Acquisition Requirements”. After this acquisi-
tion time has elapsed, the A/D conversion can be
started. An acquisition time can be programmed to
occur between setting the GO/DONE bit and the actual
start of the conversion.
6. Read A/D Result registers (ADRESH:ADRESL);
clear bit, ADIF, if required.
7. For next conversion, go to step 1 or step 2, as
required. The A/D conversion time per bit is
defined as TAD. A minimum wait of 2 TAD is
required before the next acquisition starts.
FIGURE 19-2:
A/D TRANSFER FUNCTION
The following steps should be followed to perform an A/D
conversion:
3FFh
3FEh
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 A/D module (ADCON0)
2. Configure A/D interrupt (if desired):
• Clear ADIF bit
003h
002h
001h
000h
• Set ADIE bit
• Set GIE bit
3. Wait the required acquisition time (if required).
4. Start conversion:
Analog Input Voltage
• Set GO/DONE bit (ADCON0 register)
FIGURE 19-3:
ANALOG INPUT MODEL
VDD
Sampling
Switch
VT = 0.6V
ANx
SS
RIC ≤ 1k
RSS
Rs
CPIN
VAIN
ILEAKAGE
±100 nA
CHOLD = 25 pF
VT = 0.6V
5 pF
VSS
Legend: CPIN
= Input Capacitance
= Threshold Voltage
VT
6V
5V
4V
3V
2V
ILEAKAGE = Leakage Current at the pin due to
various junctions
VDD
RIC
= Interconnect Resistance
SS
= Sampling Switch
CHOLD
RSS
= Sample/Hold Capacitance (from DAC)
= Sampling Switch Resistance
1
2
3
4
(kΩ)
Sampling Switch
© 2008 Microchip Technology Inc.
DS39631E-page 227
MICROCHIP [ MICROCHIP ]
