PIC18F258 MICROCHIP [Microchip Technology], PIC18F258 Datasheet - Page 242

• Manufacturer Part Number: PIC18F258
• Description: High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18FXX8
The value that is in the ADRESH/ADRESL registers is
not
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 con-
version is started. The analog input channels must
have their corresponding TRIS bits selected as an
input. To determine acquisition time, see Section 20.1.
After this acquisition time has elapsed, the A/D conver-
sion can be started. The following steps should be fol-
lowed for doing an A/D conversion:
1.
2.
3.
4.
5.
FIGURE 20-2:
DS41159B-page 240
Configure the A/D module:
• Configure analog pins, voltage reference and
• Select A/D input channel (ADCON0)
• Select A/D conversion clock (ADCON0)
• Turn on A/D module (ADCON0)
Configure A/D interrupt (if desired):
• Clear ADIF bit
• Set ADIE bit
• Set GIE bit
Wait the required acquisition time.
Start conversion:
• Set GO/DONE bit (ADCON0)
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
modified
digital I/O (ADCON1)
Legend: C
V
AIN
Rs
V
I
R
SS
C
for
LEAKAGE
T
PIN
IC
HOLD
ANx
ANALOG INPUT MODEL
C
5 pF
a
PIN
= input capacitance
= threshold voltage
= leakage current at the pin due to
= interconnect resistance
= sampling switch
= sample/hold capacitance (from DAC)
Power-on
various junctions
Reset.
V
DD
V
V
T
T
= 0.6V
= 0.6V
The
Preliminary
I
± 500 nA
LEAKAGE
6.
7.
20.1
For the A/D converter to meet its specified accuracy,
the charge holding capacitor (C
to fully charge to the input channel voltage level. The
analog input model is shown in Figure 20-2. The
source impedance (R
switch (R
required to charge the capacitor C
switch (R
(V
at the analog input (due to pin leakage current). The
maximum recommended impedance for analog
sources is 2.5 kΩ. After the analog input channel is
selected (changed), this acquisition must be done
before the conversion can be started.
R
IC
DD
Note:
≤ 1k
Read A/D Result registers (ADRESH/ADRESL);
clear bit ADIF if required.
For next conversion, go to step 1 or step 2, as
required. The A/D conversion time per bit is
defined as T
required before next acquisition starts.
). The source impedance affects the offset voltage
A/D Acquisition Requirements
SS
V
SS
When the conversion is started, the hold-
ing capacitor is disconnected from the
input pin.
DD
SS
) impedance varies over the device voltage
Sampling
Switch
) impedance directly affect the time
6V
5V
4V
3V
2V
R
SS
AD
. A minimum wait of 2 T
S
Sampling Switch
5 6 7 8 9 10 11
 2002 Microchip Technology Inc.
) and the internal sampling
V
SS
C
HOLD
HOLD
= 120 pF
HOLD
) must be allowed
(k Ω )
. The sampling
AD
is