PIC18CXX2_13 Datasheet, PDF (170/304 Pages) Microchip Technology – High Performance Microcontrollers with 10-bit A/D

PIC18CXX2
16.2 Selecting the A/D Conversion
Clock
The A/D conversion time per bit is defined as TAD. The
A/D conversion requires 12 TAD per 10-bit conversion.
The source of the A/D conversion clock is software
selectable. The seven possible options for TAD are:
• 2TOSC
• 4TOSC
• 8TOSC
• 16TOSC
• 32TOSC
• 64TOSC
• Internal RC oscillator
For correct A/D conversions, the A/D conversion clock
(TAD) must be selected to ensure a minimum TAD time
of 1.6 s.
Table 16-1 shows the resultant TAD times derived from
the device operating frequencies and the A/D clock
source selected.
16.3 Configuring Analog Port Pins
The ADCON1, TRISA and TRISE registers control the
operation of the A/D port pins. The port pins that are
desired as analog inputs must have their corresponding
TRIS bits set (input). If the TRIS bit is cleared (output),
the digital output level (VOH or VOL) will be converted.
The A/D operation is independent of the state of the
CHS2:CHS0 bits and the TRIS bits.
Note 1: When reading the port register, all pins con-
figured as analog input channels will read as
cleared (a low level). Pins configured as dig-
ital inputs will convert an analog input. Ana-
log levels on a digitally configured input will
not affect the conversion accuracy.
2: Analog levels on any pin that is defined as
a digital input (including the AN4:AN0
pins) may cause the input buffer to con-
sume current that is out of the devices
specification.
TABLE 16-1: TAD vs. DEVICE OPERATING FREQUENCIES
AD Clock Source (TAD)
Device Frequency
Operation ADCS2:ADCS0
40 MHz
20 MHz
5 MHz
1.25 MHz
2TOSC
000
4TOSC
100
8TOSC
001
16TOSC
101
32TOSC
010
64TOSC
110
RC
011
50 ns
100 ns
200 ns
400 ns
800 ns
1.6 s
2 - 6 s⁽¹⁾
100 ns⁽²⁾
200 ns⁽²⁾
400 ns⁽²⁾
800 ns⁽²⁾
1.6 s
3.2 s
2 - 6 s⁽¹⁾
400 ns⁽²⁾
800 ns⁽²⁾
1.6 s
3.2 s
6.4 s
12.8 s
2 - 6 s⁽¹⁾
1.6 s
3.2 s
6.4 s
12.8 s
25.6 s⁽³⁾
51.2 s⁽³⁾
2 - 6 s⁽¹⁾
Legend: Shaded cells are outside of recommended range.
Note 1: The RC source has a typical TAD time of 4 s.
2: These values violate the minimum required TAD time.
3: For faster conversion times, the selection of another clock source is recommended.
333.33 kHz
6 s
12 s
24 s⁽³⁾
48 s⁽³⁾
96 s⁽³⁾
192 s⁽³⁾
2 - 6 s⁽¹⁾
TABLE 16-2: TAD vs. DEVICE OPERATING FREQUENCIES (FOR EXTENDED, LC, DEVICES)
AD Clock Source (TAD)
Device Frequency
Operation
ADCS2:ADCS0
4 MHz
2 MHz
1.25 MHz
2TOSC
4TOSC
8TOSC
16TOSC
32TOSC
64TOSC
RC
000
500 ns⁽²⁾
1.0 s⁽²⁾
1.6 s⁽²⁾
100
1.0 s⁽²⁾
2.0 s⁽²⁾
3.2 s⁽²⁾
001
2.0 s⁽²⁾
4.0 s
6.4 s
101
4.0 s⁽²⁾
8.0 s
12.8 s
010
8.0 s
16.0 s
25.6 s⁽³⁾
110
16.0 s
32.0 s
51.2 s⁽³⁾
011
3 - 9 s^(1,4)
3 - 9 s^(1,4)
3 - 9 s^(1,4)
Legend: Shaded cells are outside of recommended range.
Note 1: The RC source has a typical TAD time of 6 s.
2: These values violate the minimum required TAD time.
3: For faster conversion times, the selection of another clock source is recommended.
333.33 kHz
6 s
12 s
24 s⁽³⁾
48 s⁽³⁾
96 s⁽³⁾
192 s⁽³⁾
3 - 9 s^(1,4)
DS39026D-page 170
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