PIC16F818/819
11.2 Selecting the A/D Conversion
Clock
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 (VOH or VOL) will be converted.
The A/D conversion time per bit is defined as TAD. The
A/D conversion requires 9.0 TAD per 10-bit conversion.
The source of the A/D conversion clock is software
selectable. The seven possible options for TAD are:
The A/D operation is independent of the state of the
CHS<2:0> bits and the TRIS bits.
• 2 TOSC
• 4 TOSC
• 8 TOSC
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.
• 16 TOSC
• 32 TOSC
• 64 TOSC
• Internal A/D module RC oscillator (2-6 µs)
For correct A/D conversions, the A/D conversion clock
(TAD) must be selected to ensure a minimum TAD time
as small as possible, but no less than 1.6 µs and not
greater than 6.4 µs.
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.
Table 11-1 shows the resultant TAD times derived from
the device operating frequencies and the A/D clock
source selected.
TABLE 11-1: TAD vs. MAXIMUM DEVICE OPERATING FREQUENCIES (STANDARD DEVICES (F))
AD Clock Source (TAD)
Maximum Device Frequency
Operation
ADCS<2>
ADCS<1:0>
2 TOSC
4 TOSC
0
1
0
1
0
1
X
00
00
01
01
10
10
11
1.25 MHz
2.5 MHz
5 MHz
8 TOSC
16 TOSC
32 TOSC
64 TOSC
RC(1,2,3)
10 MHz
20 MHz
20 MHz
(Note 1)
Note 1: The RC source has a typical TAD time of 4 µs but can vary between 2-6 µs.
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”.
2003 Microchip Technology Inc.
DS39598D-page 85
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