PIC18F1220 MICROCHIP [Microchip Technology], PIC18F1220 Datasheet - Page 17

• Manufacturer Part Number: PIC18F1220
• Description: 18/20/28-Pin High-Performance, Enhanced Flash Microcontrollers with 10-bit A/D and nanoWatt Technology
• Manufacturer: MICROCHIP [Microchip Technology]

REGISTER 2-1:
2.7
Like previous PIC18 devices, the PIC18F1220/1320
devices include a feature that allows the system clock
source to be switched from the main oscillator to an
alternate low-frequency clock source. PIC18F1220/
1320 devices offer two alternate clock sources. When
enabled, these give additional options for switching to
the various power managed operating modes.
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 on POR by the contents
of Configuration Register 1H. The details of these
modes are covered earlier in this chapter.
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.
 2004 Microchip Technology Inc.
Clock Sources and Oscillator
Switching
bit 7-6
bit 5-0
OSCTUNE: OSCILLATOR TUNING REGISTER
Unimplemented: Read as ‘0’
TUN<5:0>: Frequency Tuning bits
011111 = Maximum frequency
•
•
000001
000000 = Center frequency. Oscillator module is running at the calibrated frequency.
111111
•
•
100000 = Minimum frequency
Legend:
R = Readable bit
-n = Value at POR
bit 7
U-0
—
U-0
—
•
•
•
•
R/W-0
TUN5
W = Writable bit
‘1’ = Bit is set
R/W-0
TUN4
PIC18F1220/1320 devices offer only 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 RB6/T1OSO and RB7/T1OSI pins. Like
the LP mode oscillator circuit, loading capacitors are
also connected from each pin to ground. These pins
are also used during ICSP operations.
The Timer1 oscillator is discussed in greater detail in
Section 12.2 “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 PIC18F1220/1320 devices
are shown in Figure 2-8. See Section 12.0 “Timer1
Module” for further details of the Timer1 oscillator. See
Section 19.1 “Configuration Bits” for configuration
register details.
PIC18F1220/1320
R/W-0
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared
TUN3
R/W-0
TUN2
x = Bit is unknown
R/W-0
TUN1
DS39605C-page 15
R/W-0
TUN0
bit 0