PIC18F6520-I Datasheet, PDF (221/380 Pages) Micrel Semiconductor – 64/80-Pin High-Performance, 256 Kbit to 1 Mbit Enhanced Flash Microcontrollers with A/D

PIC18F6520/8520/6620/8620/6720/8720
19.2 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. There are seven possible options for TAD:
• 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 19-1 shows the resultant TAD times derived from
the device operating frequencies and the A/D clock
source selected.
19.3 Configuring Analog Port Pins
The ADCON1, TRISA, TRISF and TRISH registers
control the operation of the A/D port pins. The port pins
needed as analog inputs must have their correspond-
ing 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
CHS3: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 as an
analog input. Analog levels on a digitally
configured input will not affect the
conversion accuracy.
2: Analog levels on any pin defined as a dig-
ital input may cause the input buffer to
consume current out of the device’s
specification limits.
TABLE 19-1: TAD vs. DEVICE OPERATING FREQUENCIES
AD Clock Source (TAD)
Maximum Device Frequency
Operation
ADCS2:ADCS0
PIC18FXX20
PIC18LFXX20
2 TOSC
4 TOSC
8 TOSC
16 TOSC
32 TOSC
64 TOSC
RC
000
1.25 MHz
666 kHz
100
2.50 MHz
1.33 MHz
001
5.00 MHz
2.67 MHz
101
10.0 MHz
5.33 MHz
010
20.0 MHz
10.67 MHz
110
40.0 MHz
21.33 MHz
x11
—
—
 2004 Microchip Technology Inc.
DS39609B-page 219