PIC16F87XA
11.2 Selecting the A/D Conversion
Clock
11.3 Configuring Analog Port Pins
The ADCON1 and TRIS 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 conversion time per bit is defined as TAD. The
A/D conversion requires a minimum 12TAD per 10-bit
conversion. The source of the A/D conversion clock is
software selected. The seven possible options for TAD
are:
The A/D operation is independent of the state of the
CHS2:CHS0 bits and the TRIS bits.
• 2TOSC
• 4TOSC
Note 1: When reading the port register, any pin
configured as an analog input channel 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 conver-
sion accuracy.
• 8TOSC
• 16TOSC
• 32TOSC
• 64TOSC
• 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
of 1.6 µs.
2: Analog levels on any pin that is defined as
a digital input (including the AN7:AN0
pins), may cause the input buffer to con-
sume current that is out of the device
specifications.
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 (C))
AD Clock Source (TAD)
ADCS2:ADCS1:ADCS0
Maximum Device Frequency
Max.
Operation
2TOSC
4TOSC
000
100
001
101
010
110
x11
1.25 MHz
2.5 MHz
5 MHz
8TOSC
16TOSC
32TOSC
64TOSC
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 recom-
mended for SLEEP operation.
3: For extended voltage devices (LC), please refer to the Electrical Characteristics (Sections 17.1 and 17.2).
2001 Microchip Technology Inc.
Advance Information
DS39582A-page 129
MICROCHIP [ MICROCHIP ]
