PIC18FXX20
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:
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 (out-
put), the digital output level (VOH or VOL) will be
converted.
• 2 TOSC
• 4 TOSC
• 8 TOSC
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 an ana-
log input. Analog levels on a digitally
configured input will not affect the
conversion accuracy.
• 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.
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
100
001
101
010
110
x11
1.25 MHz
2.50 MHz
5.00 MHz
10.0 MHz
20.0 MHz
40.0 MHz
—
666 kHz
1.33 MHz
2.67 MHz
5.33 MHz
10.67 MHz
21.33 MHz
—
2003 Microchip Technology Inc.
Advance Information
DS39609A-page 219
MICROCHIP [ MICROCHIP ]
