
PIC16F87X
DS30292B-page 116
1999 Microchip Technology Inc.
11.2
Selecting the A/D Conversion Clock
The A/D conversion time per bit is defined as T
AD
. The
A/D conversion requires a minimum 12T
AD
per 10-bit
conversion. The source of the A/D conversion clock is
software selected. The four possible options for T
AD
are:
2T
OSC
8T
OSC
32T
OSC
Internal RC oscillator
For correct A/D conversions, the A/D conversion clock
(T
AD
) must be selected to ensure a minimum T
AD
time
of 1.6
μ
s.
Table 11-1shows the resultant T
AD
times derived from
the device operating frequencies and the A/D clock
source selected.
TABLE 11-1:
T
AD
vs. MAXIMUM DEVICE OPERATING FREQUENCIES (STANDARD DEVICES (C))
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 (V
OH
or V
OL
) will be converted.
The A/D operation is independent of the state of the
CHS2:CHS0 bits and the TRIS bits.
AD Clock Source (T
AD
)
Maximum Device Frequency
Operation
ADCS1:ADCS0
Max.
2T
OSC
8T
OSC
32T
OSC
RC
(1, 2, 3)
00
1.25 MHz
5 MHz
20 MHz
Note 1
01
10
11
Note 1:
The RC source has a typical T
AD
time of 4
μ
s but can vary between 2-6
μ
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 (LC), please refer to the Electrical Specifications section.
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.
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.