PIC18F2420/2520/4420/4520
The secondary oscillators are those external sources
not connected to the OSC1 or OSC2 pins. These
sources may continue to operate even after the
controller is placed in a power-managed mode.
2.7
Clock Sources and Oscillator
Switching
Like previous PIC18 devices, the PIC18F2420/2520/
4420/4520 family includes a feature that allows the
device clock source to be switched from the main oscil-
lator to an alternate low-frequency clock source.
PIC18F2420/2520/4420/4520 devices offer two alternate
clock sources. When an alternate clock source is enabled,
the various power-managed operating modes are
available.
PIC18F2420/2520/4420/4520 devices offer the Timer1
oscillator as a secondary oscillator. This oscillator, in all
power-managed modes, is often the time base for
functions such as a Real-Time Clock (RTC).
Most often, a 32.768 kHz watch crystal is connected
between the RC0/T1OSO/T13CKI and RC1/T1OSI
pins. Like the LP Oscillator mode circuit, loading
capacitors are also connected from each pin to ground.
Essentially, there are three clock sources for these
devices:
The Timer1 oscillator is discussed in greater detail in
Section 12.3 “Timer1 Oscillator”.
• Primary oscillators
• Secondary oscillators
• Internal oscillator block
In addition to being a primary clock source, the internal
oscillator block is available as a power-managed
mode clock source. The INTRC source is also used as
the clock source for several special features, such as
the WDT and Fail-Safe Clock Monitor.
The primary oscillators include the External Crystal
and Resonator modes, the External RC modes, the
External Clock modes and the internal oscillator block.
The particular mode is defined by the FOSC<3:0> Con-
figuration bits. The details of these modes are covered
earlier in this chapter.
The clock sources for the PIC18F2420/2520/4420/4520
devices are shown in Figure 2-8. See Section 23.0
“Special Features of the CPU” for Configuration
register details.
FIGURE 2-8:
PIC18F2420/2520/4420/4520 CLOCK DIAGRAM
PIC18F2420/2520/4420/4520
Primary Oscillator
LP, XT, HS, RC, EC
HSPLL, INTOSC/PLL
T1OSC
OSC2
Sleep
4 x PLL
OSC1
OSCTUNE<6>
Peripherals
Secondary Oscillator
T1OSO
T1OSCEN
Enable
Oscillator
T1OSI
OSCCON<6:4>
Internal Oscillator
CPU
8 MHz
OSCCON<6:4>
111
110
101
4 MHz
2 MHz
Internal
Oscillator
Block
IDLEN
Clock
Control
1 MHz
100
011
010
001
000
8 MHz
Source
500 kHz
250 kHz
125 kHz
31 kHz
8 MHz
(INTOSC)
INTRC
Source
FOSC<3:0> OSCCON<1:0>
Clock Source Option
for Other Modules
1
0
31 kHz (INTRC)
OSCTUNE<7>
WDT, PWRT, FSCM
and Two-Speed Start-up
DS39631E-page 28
© 2008 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
