PIC18F258 Datasheet, PDF (244/384 Pages) Microchip Technology – High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN

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 con-
figured as analog input channels will read
as cleared (a low level). Pins configured 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 con-
sume current that is out of the devices
specification.
TABLE 20-1: TAD vs. DEVICE OPERATING FREQUENCIES
AD Clock Source (TAD)
Device Frequency
Operation
ADCS2:ADCS0
20 MHz
5 MHz
1.25 MHz
2 TOSC
4 TOSC
8 TOSC
16 TOSC
32 TOSC
64 TOSC
RC
000
100 ns⁽²⁾
400 ns⁽²⁾
1.6 µs
100
200 ns⁽²⁾
800 ns⁽²⁾
3.2 µs
001
400 ns⁽²⁾
1.6 µs
6.4 µs
101
800 ns⁽²⁾
3.2 µs
12.8 µs
010
1.6 µs
6.4 µs
25.6 µs⁽³⁾
110
3.2 µs
12.8 µs
51.2 µs⁽³⁾
011
2 - 6 µs⁽¹⁾
2 - 6 µs⁽¹⁾
2 - 6 µs⁽¹⁾
Legend: Shaded cells are outside of recommended range.
Note 1: The RC source has a typical TAD time of 4 µs.
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 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, LC DEVICES)
AD Clock Source (TAD)
Device Frequency
Operation
ADCS2:ADCS0
4 MHz
2 MHz
1.25 MHz
2 TOSC
4 TOSC
8 TOSC
16 TOSC
32 TOSC
64 TOSC
RC
000
500 ns⁽²⁾
1.0 µs⁽²⁾
1.6 µs⁽²⁾
100
1.0 µs⁽²⁾
2.0 µs⁽²⁾
3.2 µs⁽²⁾
001
2.0 µs⁽²⁾
4.0 µs
6.4 µs
101
4.0 µs⁽²⁾
8.0 µs
12.8 µs
010
8.0 µs
16.0 µs
25.6 µs⁽³⁾
110
16.0 µs
32.0 µs
51.2 µs⁽³⁾
011
3 - 9 µs^(1,4)
3 - 9 µs^(1,4)
3 - 9 µs^(1,4)
Legend: Shaded cells are outside of recommended range.
Note 1: The RC source has a typical TAD time of 6 µs.
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 µs
12 µs
24 µs⁽³⁾
48 µs⁽³⁾
96 µs⁽³⁾
192 µs⁽³⁾
3 - 9 µs^(1,4)
DS41159B-page 242
Preliminary
 2002 Microchip Technology Inc.