Datasheet
PIC16F87/88
DS30487D-page 118 2002-2013 Microchip Technology Inc.
12.2 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 12-1 shows the resultant T
AD times derived from
the device operating frequencies and the A/D clock
source selected.
12.3 Operation in Power-Managed
Modes
The selection of the automatic acquisition time and
A/D conversion clock is determined in part by the clock
source and frequency while in a power-managed
mode.
If the A/D is expected to operate while the device is in
a power-managed mode, the ADCS2:ADCS0 bits in
ADCON0 and ADCON1 should be updated in accor-
dance with the power-managed mode clock that will be
used. After the power-managed mode is entered
(either of the power-managed Run modes), an A/D
acquisition or conversion may be started. Once an
acquisition or conversion is started, the device should
continue to be clocked by the same power-managed
mode clock source until the conversion has been
completed.
If the power-managed mode clock frequency is less
than 1 MHz, the A/D RC clock source should be
selected.
TABLE 12-1: TAD vs. MAXIMUM DEVICE OPERATING FREQUENCIES – STANDARD DEVICES (C)
AD Clock Source (TAD) Maximum Device Frequency
Operation ADCS<2> ADCS<1:0> Max.
2 T
OSC 0001.25 MHz
4 T
OSC 1002.5 MHz
8 TOSC 001 5 MHz
16 TOSC 101 10 MHz
32 T
OSC 010 20 MHz
64 TOSC 110 20 MHz
RC
(1,2,3)
x11(Note 1)
Note 1: The RC source has a typical T
AD 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 18.0 “Electrical Characteristics”.