PIC18F2450/4450
The value in the ADRESH:ADRESL registers is not
modified for a Power-on Reset. The ADRESH:ADRESL
registers will contain unknown data after a Power-on
Reset.
5. Wait for A/D conversion to complete, by either:
• Polling for the GO/DONE bit to be cleared
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 conver-
sion is started. The analog input channels must have
their corresponding TRIS bits selected as an input. To
determine acquisition time, see Section 16.1 “A/D
Acquisition Requirements”. After this acquisition 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 3 TAD is
required before the next acquisition starts.
FIGURE 16-2:
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 16-3:
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
= Sampling Switch
SS
CHOLD
RSS
= Sample/hold Capacitance (from DAC)
= Sampling Switch Resistance
1
2
3
4
(kΩ)
Sampling Switch
© 2006 Microchip Technology Inc.
Advance Information
DS39760A-page 177
MICROCHIP [ MICROCHIP ]
