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PIC16(L)F1508/9
DS40001609E-page 50
2011-2015 Microchip Technology Inc.
5.2.1.4
Secondary Oscillator
The secondary oscillator is a separate crystal oscillator
that is associated with the Timer1 peripheral. It is opti-
mized for timekeeping operations with a 32.768 kHz
crystal connected between the SOSCO and SOSCI
device pins.
The secondary oscillator can be used as an alternate
system clock source and can be selected during
run-time using clock switching. Refer to
for more information.
FIGURE 5-5:
QUARTZ CRYSTAL
OPERATION
(SECONDARY
OSCILLATOR)
5.2.1.5
External RC Mode
The External Resistor-Capacitor (EXTRC) mode
supports the use of an external RC circuit. This allows the
designer maximum flexibility in frequency choice while
keeping costs to a minimum when clock accuracy is not
required.
The RC circuit connects to OSC1. OSC2/CLKOUT is
available for general purpose I/O or CLKOUT. The
function of the OSC2/CLKOUT pin is determined by the
CLKOUTEN bit in Configuration Words.
shows the External RC mode connections.
FIGURE 5-6:
EXTERNAL RC MODES
The RC oscillator frequency is a function of the supply
voltage, the resistor (R
EXT
) and capacitor (C
EXT
) values
and the operating temperature. Other factors affecting
the oscillator frequency are:
• threshold voltage variation
• component tolerances
• packaging variations in capacitance
The user also needs to take into account variation due
to tolerance of the external RC components used.
Note 1:
Quartz crystal characteristics vary
according to type, package and
manufacturer. The user should consult the
manufacturer data sheets for specifications
and recommended application.
2:
Always verify oscillator performance over
the V
DD
and temperature range that is
expected for the application.
3:
For oscillator design assistance, reference
the following Microchip Applications Notes:
• AN826, “
Crystal Oscillator Basics and
Crystal Selection for rfPIC
®
and PIC
®
Devices
” (DS00826)
• AN849, “
Basic PIC
®
Oscillator Design
”
(DS00849)
• AN943, “
Practical PIC
®
Oscillator
Analysis and Design
” (DS00943)
• AN949, “
Making Your Oscillator Work
”
(DS00949)
• TB097, “
Interfacing a Micro Crystal
MS1V-T1K 32.768 kHz Tuning Fork
Crystal to a PIC16F690/SS
” (DS91097)
• AN1288, “
Design Practices for
Low-Power External Oscillators
”
(DS01288)
SOSCI
PIC
®
MCU
SOSCO
C2
C1
To Internal
Logic
32.768 kHz
Quartz
Crystal
Rev. 10-000061A
7/30/2013
F
OSC
/4
or I/O
(1)
PIC
®
MCU
OSC2/CLKOUT
Note 1:
Output depends upon the CLKOUTEN bit of the
Configuration Words.
OSC1/CLKIN
Internal
Clock
V
SS
C
EXT
R
EXT
Recommended values:10 k
R
EXT
100 k
, <3V
3 k
R
EXT
100 k
, 3-5V
C
EXT
> 20 pF, 2-5V
Rev. 10-000062A
7/31/2013
V
DD
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