PIC18F2450 Datasheet, PDF (179/320 Pages) Microchip Technology – 28/40/44-Pin, High-Performance, 12 MIPS, Enhanced Flash, USB Microcontrollers with nanoWatt Technology

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.
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.
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 A/D module (ADCON0)
2. Configure 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:
• Set GO/DONE bit (ADCON0 register)
PIC18F2450/4450
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
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
3FFh
3FEh
003h
002h
001h
000h
Analog Input Voltage
FIGURE 16-3:
ANALOG INPUT MODEL
VDD
Rs ^ANx
VT = 0.6V
RIC ≤ 1k
Sampling
Switch
SS _RSS
VAIN
CPIN
5 pF
VT = 0.6V
ILEAKAGE
±100 nA
CHOLD = 25 pF
VSS
Legend: CPIN
VT
ILEAKAGE
RIC
SS
CHOLD
RSS
= Input Capacitance
= Threshold Voltage
= Leakage Current at the pin due to
various junctions
= Interconnect Resistance
= Sampling Switch
= Sample/hold Capacitance (from DAC)
= Sampling Switch Resistance
6V
5V
VDD 4V
3V
2V
1 23 4
Sampling Switch (kΩ)
© 2006 Microchip Technology Inc.
Advance Information
DS39760A-page 177