PIC16F87_05 Datasheet, PDF (120/228 Pages) Microchip Technology – 18/20/28-Pin Enhanced Flash Microcontrollers with nanoWatt Technology

PIC16F87/88
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 TAD per 10-bit conversion.
The source of the A/D conversion clock is software
selectable. The seven possible options for TAD are:
• 2 TOSC
• 4 TOSC
• 8 TOSC
• 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.
Table 12-1 shows the resultant TAD 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 TOSC
0
00
1.25 MHz
4 TOSC
1
00
2.5 MHz
8 TOSC
0
01
5 MHz
16 TOSC
1
01
10 MHz
32 TOSC
0
10
20 MHz
64 TOSC
1
10
20 MHz
RC^(1,2,3)
x
11
(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 18.0 “Electrical Characteristics”.
DS30487C-page 118
 2005 Microchip Technology Inc.