Datasheet

 2001-2013 Microchip Technology Inc. DS39598F-page 85
PIC16F818/819
11.2 Selecting the A/D Conversion
Clock
The A/D conversion time per bit is defined as TAD. The
A/D conversion requires 9.0 T
AD per 10-bit conversion.
The source of the A/D conversion clock is software
selectable. The seven possible options for T
AD are:
•2T
OSC
•4TOSC
•8TOSC
•16TOSC
•32TOSC
•64TOSC
• Internal A/D module RC oscillator (2-6 s)
For correct A/D conversions, the A/D conversion clock
(T
AD) must be selected to ensure a minimum TAD time
as small as possible, but no less than 1.6 s and not
greater than 6.4 s.
Table 11-1 shows the resultant T
AD times derived from
the device operating frequencies and the A/D clock
source selected.
11.3 Configuring Analog Port Pins
The ADCON1 and TRISA registers control the opera-
tion 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 (V
OH or VOL) will be converted.
The A/D operation is independent of the state of the
CHS<2:0> bits and the TRIS bits.
TABLE 11-1: TAD vs. MAXIMUM DEVICE OPERATING FREQUENCIES (STANDARD DEVICES (F))
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 out of the device
specification.
AD Clock Source (TAD)
Maximum Device Frequency
Operation ADCS<2> ADCS<1:0>
2 TOSC 0001.25 MHz
4 TOSC 1002.5 MHz
8 TOSC 001 5 MHz
16 T
OSC 101 10 MHz
32 TOSC 010 20 MHz
64 TOSC 110 20 MHz
RC
(1,2,3)
X11(Note 1)
Note 1: The RC source has a typical TAD time of 4 s but can vary between 2-6 s.
2: When the device frequencies are greater than 1 MHz, the RC A/D conversion clock source is only
recommended for Sleep operation.
3: For extended voltage devices (LF), please refer to Section 15.0 “Electrical Characteristics”.