Microchip Technology MA160014 Data Sheet
PIC18(L)F2X/4XK22
DS41412E-page 34
2010-2012 Microchip Technology Inc.
2.4
Clock Source Modes
Clock Source modes can be classified as external or
internal.
internal.
• External Clock modes rely on external circuitry for
the clock source. Examples are: Clock modules
(EC mode), quartz crystal resonators or ceramic
resonators (LP, XT and HS modes) and Resistor-
Capacitor (RC mode) circuits.
(EC mode), quartz crystal resonators or ceramic
resonators (LP, XT and HS modes) and Resistor-
Capacitor (RC mode) circuits.
• Internal clock sources are contained internally
within the Oscillator block. The Oscillator block
has three internal oscillators: the 16 MHz High-
Frequency Internal Oscillator (HFINTOSC),
500 kHz Medium-Frequency Internal Oscillator
(MFINTOSC) and the 31.25 kHz Low-Frequency
Internal Oscillator (LFINTOSC).
has three internal oscillators: the 16 MHz High-
Frequency Internal Oscillator (HFINTOSC),
500 kHz Medium-Frequency Internal Oscillator
(MFINTOSC) and the 31.25 kHz Low-Frequency
Internal Oscillator (LFINTOSC).
The system clock can be selected between external or
internal clock sources via the System Clock Select
(SCS<1:0>) bits of the OSCCON register. See
internal clock sources via the System Clock Select
(SCS<1:0>) bits of the OSCCON register. See
for additional
information.
2.5
External Clock Modes
2.5.1
OSCILLATOR START-UP TIMER (OST)
When 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. When switching between clock sources, a
delay is required to allow the new clock to stabilize.
These oscillator delays are shown in
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. When switching between clock sources, a
delay is required to allow the new clock to stabilize.
These oscillator delays are shown in
In order to minimize latency between external oscillator
start-up and code execution, the Two-Speed Clock
Start-up mode can be selected (see
start-up and code execution, the Two-Speed Clock
Start-up mode can be selected (see
).
TABLE 2-2:
OSCILLATOR DELAY EXAMPLES
2.5.2
EC MODE
The External Clock (EC) mode allows an externally
generated logic level as the system clock source. When
operating in this mode, an external clock source is
connected to the OSC1 input and the OSC2 is available
for general purpose I/O.
generated logic level as the system clock source. When
operating in this mode, an external clock source is
connected to the OSC1 input and the OSC2 is available
for general purpose I/O.
shows the pin
connections for EC mode.
The External Clock (EC) offers different power modes,
Low Power (ECLP), Medium Power (ECMP) and High
Power (ECHP), selectable by the FOSC<3:0> bits.
Each mode is best suited for a certain range of
frequencies. The ranges are:
Low Power (ECLP), Medium Power (ECMP) and High
Power (ECHP), selectable by the FOSC<3:0> bits.
Each mode is best suited for a certain range of
frequencies. The ranges are:
• ECLP – below 500 kHz
• ECMP – between 500 kHz and 16 MHz
• ECHP – above 16 MHz
• ECMP – between 500 kHz and 16 MHz
• ECHP – above 16 MHz
The Oscillator Start-up Timer (OST) is disabled when
EC mode is selected. Therefore, there is no delay in
operation after a Power-on Reset (POR) or wake-up
from Sleep. Because the PIC
EC mode is selected. Therefore, there is no delay in
operation after a Power-on Reset (POR) or wake-up
from Sleep. Because the PIC
®
MCU design is fully
static, stopping the external clock input will have the
effect of halting the device while leaving all data intact.
Upon restarting the external clock, the device will
resume operation as if no time had elapsed.
Upon restarting the external clock, the device will
resume operation as if no time had elapsed.
FIGURE 2-5:
EXTERNAL CLOCK (EC)
MODE OPERATION
MODE OPERATION
Switch From
Switch To
Frequency
Oscillator Delay
Sleep/POR
LFINTOSC
MFINTOSC
HFINTOSC
31.25 kHz
31.25 kHz to 500 kHz
31.25 kHz to 16 MHz
Oscillator Warm-Up Delay (T
WARM
)
Sleep/POR
EC, RC
DC – 64 MHz
2 instruction cycles
LFINTOSC (31.25 kHz)
EC, RC
DC – 64 MHz
1 cycle of each
Sleep/POR
LP, XT, HS
32 kHz to 40 MHz
1024 Clock Cycles (OST)
Sleep/POR
4xPLL
32 MHz to 64 MHz
1024 Clock Cycles (OST) + 2 ms
LFINTOSC (31.25 kHz)
LFINTOSC
HFINTOSC
31.25 kHz to 16 MHz
1
s (approx.)
OSC1/CLKIN
OSC2/CLKOUT
(1)
I/O
Clock from
Ext. System
Ext. System
PIC
®
MCU
Note
1:
Alternate pin functions are listed in