PIC18F2420 MICROCHIP [Microchip Technology], PIC18F2420 Datasheet - Page 30

• Manufacturer Part Number: PIC18F2420
• Description: 28/40/44-Pin Enhanced Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18F2420/2520/4420/4520
2.7
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 avail-
able.
Essentially, there are three clock sources for these
devices:
• Primary oscillators
• Secondary oscillators
• Internal oscillator block
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 FOSC3:FOSC0
configuration bits. The details of these modes are
covered earlier in this chapter.
FIGURE 2-8:
DS39631A-page 28
T1OSO
T1OSI
OSC2
OSC1
Clock Sources and Oscillator
Switching
Primary Oscillator
Secondary Oscillator
PIC18F2420/2520/4420/4520 CLOCK DIAGRAM
OSCCON<6:4>
Oscillator
Internal
Source
INTRC
Source
8 MHz
Block
T1OSCEN
Enable
Oscillator
Sleep
31 kHz (INTRC)
(INTOSC)
8 MHz
OSCTUNE<6>
PIC18F2420/2520/4420/4520
Preliminary
1
0
500 kHz
250 kHz
125 kHz
31 kHz
8 MHz
4 MHz
2 MHz
1 MHz
OSCCON<6:4>
register details.
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.
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.
Most often, a 32.768 kHz watch crystal is connected
between the RC0/T1OSO/T13CKI and RC1/T1OSI
pins. Like the LP mode oscillator circuit, loading
capacitors are also connected from each pin to ground.
The Timer1 oscillator is discussed in greater detail in
Section 12.3 “Timer1 Oscillator”.
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 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
OSCTUNE<7>
111
110
101
100
011
010
001
000
4 x PLL
HSPLL, INTOSC/PLL
LP, XT, HS, RC, EC
Internal Oscillator
FOSC3:FOSC0
T1OSC
 2004 Microchip Technology Inc.
Clock Source Option
for other Modules
WDT, PWRT, FSCM
and Two-Speed Start-up
Control
Clock
OSCCON<1:0>
Peripherals
IDLEN
CPU