Datasheet

PIC18F2XK20/4XK20
DS41303G-page 30  2010 Microchip Technology Inc.
2.3 Clock Source Modes
Clock Source modes can be classified as external or
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.
• Internal clock sources are contained internally
within the Oscillator block. The Oscillator block
has two internal oscillators: the 16 MHz High-
Frequency Internal Oscillator (HFINTOSC) and
the 31 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
Section 2.9 “Clock Switching” for additional informa-
tion.
2.4 External Clock Modes
2.4.1 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 Table 2-1.
In order to minimize latency between external oscillator
start-up and code execution, the Two-Speed Clock
Start-up mode can be selected (see Section 2.10
“Two-Speed Clock Start-up Mode”).
TABLE 2-1: OSCILLATOR DELAY EXAMPLES
2.4.2 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. Figure 2-2 shows the pin
connections for EC mode.
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
®
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.
FIGURE 2-2: EXTERNAL CLOCK (EC)
MODE OPERATION
Switch From Switch To Frequency Oscillator Delay
Sleep/POR
LFINTOSC
HFINTOSC
31 kHz
250 kHz to 16 MHz
Oscillator Warm-Up Delay (T
WARM)
Sleep/POR EC, RC DC – 64 MHz 2 instruction cycles
LFINTOSC (31 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 HSPLL 32 MHz to 64 MHz 1024 Clock Cycles (OST) + 2 ms
LFINTOSC (31 kHz) HFINTOSC 250 kHz to 16 MHz 1 s (approx.)
OSC1/CLKIN
OSC2/CLKOUT
(1)
I/O
Clock from
Ext. System
PIC
®
MCU
Note 1: Alternate pin functions are listed in
Section 1.0 “Device Overview”.