PIC18F1220ESLASHSOSQTP datasheet(17/308 Pages) MICROCHIP

17 / 308 page

DS39605F-page 15

PIC18F1220/1320

OSCTUNE: OSCILLATOR TUNING REGISTER

2.7

Clock Sources and Oscillator

Switching

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.

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

U-0

R/W-0

—

TUN5

TUN4

TUN3

TUN2

TUN1

TUN0

bit 7

bit 0

bit 7-6

Unimplemented: Read as ‘0’

bit 5-0

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

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

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