Datasheet
2001 Microchip Technology Inc. DS30292C-page 115
PIC16F87X
11.2 Selecting the A/D Conversion
Clock
The A/D conversion time per bit is defined as TAD. 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
• 8TOSC
• 32TOSC
• Internal A/D module RC oscillator (2-6 µs)
For correct A/D conversions, the A/D conversion clock
(T
AD) must be selected to ensure a minimum TAD time
of 1.6 µs.
Table 11-1 shows the resultant T
AD 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))
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 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 00 1.25 MHz
8T
OSC 01 5 MHz
32TOSC 10 20 MHz
RC
(1, 2, 3)
11 (Note 1)
Note 1: The RC source has a typical T
AD 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 15.1 and 15.2).
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.