PIC18FXX8 Datasheet, PDF (248/402 Pages) Microchip Technology – 28/40-Pin High-Performance, Enhanced Flash Microcontrollers with CAN Module

PIC18FXX8
20.2 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:
• 2 TOSC
• 4 TOSC
• 8 TOSC
• 16 TOSC
• 32 TOSC
• 64 TOSC
• 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 µs.
Table 20-1 shows the resultant TAD times derived from
the device operating frequencies and the A/D clock
source selected.
20.3 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
configured as analog input channels will
read as cleared (a low level). Pins config-
ured as digital inputs will convert an
analog input. Analog 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
consume current that is out of the
device’s specification.
TABLE 20-1: TAD vs. DEVICE OPERATING FREQUENCIES
AD Clock Source (TAD)
Device Frequency
Operation
2 TOSC
4 TOSC
8 TOSC
16 TOSC
32 TOSC
64 TOSC
RC
ADCS2:ADCS0
000
100
001
101
010
110
011
20 MHz
100 ns⁽²⁾
200 ns⁽²⁾
400 ns⁽²⁾
800 ns⁽²⁾
1.6 µs
3.2 µs
2-6 µs⁽¹⁾
5 MHz
400 ns⁽²⁾
800 ns⁽²⁾
1.6 µs
3.2 µs
6.4 µs
12.8 µs
2-6 µs⁽¹⁾
1.25 MHz
1.6 µs
3.2 µs
6.4 µs
12.8 µs
25.6 µs⁽³⁾
51.2 µs⁽³⁾
2-6 µs⁽¹⁾
Legend:
Note 1:
2:
3:
Shaded cells are outside of recommended range.
The RC source has a typical TAD time of 4 µs.
These values violate the minimum required TAD time.
For faster conversion times, the selection of another clock source is recommended.
333.33 kHz
6 µs
12 µs
24 µs⁽³⁾
48 µs⁽³⁾
96 µs⁽³⁾
192 µs⁽³⁾
2-6 µs⁽¹⁾
TABLE 20-2: TAD vs. DEVICE OPERATING FREQUENCIES (FOR EXTENDED, LF DEVICES)
AD Clock Source (TAD)
Device Frequency
Operation
2 TOSC
4 TOSC
8 TOSC
16 TOSC
32 TOSC
64 TOSC
RC
ADCS2:ADCS0
000
100
001
101
010
110
011
4 MHz
500 ns⁽²⁾
1.0 µs⁽²⁾
2.0 µs⁽²⁾
4.0 µs⁽²⁾
8.0 µs
16.0 µs
3-9 µs⁽¹⁾
2 MHz
1.0 µs⁽²⁾
2.0 µs⁽²⁾
4.0 µs
8.0 µs
16.0 µs
32.0 µs
3-9 µs⁽¹⁾
1.25 MHz
1.6 µs⁽²⁾
3.2 µs⁽²⁾
6.4 µs
12.8 µs
25.6 µs⁽³⁾
51.2 µs⁽³⁾
3-9 µs⁽¹⁾
333.33 kHz
6 µs
12 µs
24 µs⁽³⁾
48 µs⁽³⁾
96 µs⁽³⁾
192 µs⁽³⁾
3-9 µs⁽¹⁾
Legend:
Note 1:
2:
3:
Shaded cells are outside of recommended range.
The RC source has a typical TAD time of 6 µs.
These values violate the minimum required TAD time.
For faster conversion times, the selection of another clock source is recommended.
DS41159D-page 246
 2004 Microchip Technology Inc.