
PIC12(L)F1840
DS41441B-page 50
Preliminary
2011 Microchip Technology Inc.
FIGURE 5-3:
QUARTZ CRYSTAL
OPERATION (LP, XT OR
HS MODE)
FIGURE 5-4:
CERAMIC RESONATOR
OPERATION
(XT OR HS MODE)
5.2.1.3
Oscillator Start-up Timer (OST)
If the oscillator module is configured for LP, XT or HS
modes, the Oscillator Start-up Timer (OST) counts
1024 oscillations from OSC1. This occurs following a
Power-on Reset (POR) and when the Power-up Timer
(PWRT) has expired (if configured), or a wake-up from
Sleep. During this time, the program counter does not
increment and program execution is suspended. The
OST ensures that the oscillator circuit, using a quartz
crystal resonator or ceramic resonator, has started and
is providing a stable system clock to the oscillator
module.
In order to minimize latency between external oscillator
start-up and code execution, the Two-Speed Clock
5.2.1.4
4X PLL
The oscillator module contains a 4X PLL that can be
used with both external and internal clock sources to
provide a system clock source. The input frequency for
the 4X PLL must fall within specifications. See the PLL
Clock
Timing
Specifications
in
The 4X PLL may be enabled for use by one of two
methods:
1.
Program the PLLEN bit in Configuration Word 2
to a ‘1’.
2.
Write the SPLLEN bit in the OSCCON register to
a ‘1’. If the PLLEN bit in Configuration Word 2 is
programmed to a ‘1’, then the value of SPLLEN
is ignored.
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 VDD 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)
Note 1:
A series resistor (RS) may be required for
quartz crystals with low drive level.
2:
The value of RF varies with the Oscillator mode
selected (typically between 2 M
to 10 M.
C1
C2
Quartz
RS(1)
OSC1/CLKIN
RF(2)
Sleep
To Internal
Logic
PIC MCU
Crystal
OSC2/CLKOUT
Note 1:
A series resistor (RS) may be required for
ceramic resonators with low drive level.
2:
The value of RF varies with the Oscillator mode
selected (typically between 2 M
to 10 M.
3:
An additional parallel feedback resistor (RP)
may be required for proper ceramic resonator
operation.
C1
C2 Ceramic
RS(1)
OSC1/CLKIN
RF(2)
Sleep
To Internal
Logic
PIC MCU
RP(3)
Resonator
OSC2/CLKOUT