2009 Microchip Technology Inc.
DS39887C-page 25
PIC18F2458/2553/4458/4553
The value in the ADRESH:ADRESL registers is
unknown following Power-on and Brown-out 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 an
input. To determine acquisition time, see Section 2.1
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.
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)
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 2 TAD is
required before the next acquisition starts.
FIGURE 2-2:
A/D TRANSFER FUNCTION
FIGURE 2-3:
ANALOG INPUT MODEL
Dig
ita
lCo
d
eO
utp
ut
FFEh
003h
002h
001h
000h
0
.5
LS
B
1LS
B
1
.5
LS
B
2LS
B
2
.5
LS
B
40
94
LS
B
4
094
.5
LS
B
3LS
B
Analog Input Voltage
FFFh
40
95
LS
B
4
095
.5
LS
B
VAIN
CPIN
Rs
ANx
5 pF
VT = 0.6V
ILEAKAGE
RIC
≤ 1k
Sampling
Switch
SS
RSS
CHOLD = 25 pF
VSS
VDD
±100 nA
Legend: CPIN
VT
ILEAKAGE
RIC
SS
CHOLD
= Input Capacitance
= Threshold Voltage
= Leakage Current at the pin due to
= Interconnect Resistance
= Sampling Switch
= Sample/Hold Capacitance (from DAC)
various junctions
= Sampling Switch Resistance
RSS
VDD
6V
Sampling Switch
5V
4V
3V
2V
12
3
4
(k
Ω)
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