PIC16F87 datasheet(39/230 Pages) MICROCHIP

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DS30487D-page 39

PIC16F87/88

4.6

Clock Sources and Oscillator

Switching

The PIC16F87/88 devices include a feature that allows

the system clock source to be switched from the main

oscillator to an alternate low-frequency clock source.

PIC16F87/88 devices offer three 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 (INTRC)

The primary oscillators include the External Crystal

and Resonator modes, the External RC modes, the

External Clock mode and the internal oscillator block.

The particular mode is defined on POR by the contents

of Configuration Word 1. 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.

PIC16F87/88 devices offer the Timer1 oscillator as a

secondary oscillator. This oscillator continues to run

when a SLEEP instruction is executed and 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. The Timer1

oscillator is discussed in greater detail in Section 7.6

“Timer1 Oscillator”.

In addition to being a primary clock source, the internal

oscillator block is available as a power-managed

mode clock source. The 31.25 kHz INTRC source is

also used as the clock source for several special

features, such as the WDT, Fail-Safe Clock Monitor,

Power-up Timer and Two-Speed Start-up.

The clock sources for the PIC16F87/88 devices are

shown in Figure 4-6. See Section 7.0 “Timer1 Mod-

ule” for further details of the Timer1 oscillator. See

Section 15.1 “Configuration Bits” for Configuration

4.6.1

OSCCON REGISTER

The OSCCON register (Register 4-2) controls several

aspects of the system clock’s operation, both in full

power operation and in power-managed modes.

The System Clock Select bits, SCS1:SCS0, select the

clock source that is used when the device is operating

power-managed

modes.

When

the

bits

are

cleared (SCS<1:0> = 00), the system clock source

comes from the main oscillator that is selected by the

FOSC2:FOSC0 configuration bits in Configuration

Word 1 register. When the bits are set in any other

manner, the system clock source is provided by the

Timer1 oscillator (SCS1:SCS0 = 01) or from the

internal oscillator block (SCS1:SCS0 = 10). After a

Reset, SCS<1:0> are always set to ‘00’.

The Internal Oscillator Select bits, IRCF2:IRCF0, select

the frequency output of the internal oscillator block that

is used to drive the system clock. The choices are the

INTRC source (31.25 kHz), the INTOSC source

(8 MHz) or one of the six frequencies derived from the

INTOSC postscaler (125 kHz to 4 MHz). Changing the

configuration of these bits has an immediate change on

the multiplexor’s frequency output.

The OSTS and IOFS bits indicate the status of the

primary oscillator and INTOSC source; these bits are

set when their respective oscillators are stable. In

particular, OSTS indicates that the Oscillator Start-up

Timer has timed out.

4.6.2

CLOCK SWITCHING

Clock switching will occur for the following reasons:

• The FCMEN (CONFIG2<0>) bit is set, the device

is running from the primary oscillator and the

primary oscillator fails. The clock source will be

the internal RC oscillator.

• The FCMEN bit is set, the device is running from

the T1OSC and T1OSC fails. The clock source

will be the internal RC oscillator.

• Following a wake-up due to a Reset or a POR,

when the device is configured for Two-Speed

Start-up mode, switching will occur between the

INTRC and the system clock defined by the

FOSC<2:0> bits.

• A wake-up from Sleep occurs due to an interrupt or

WDT wake-up and Two-Speed Start-up is enabled.

If the primary clock is XT, HS or LP, the clock will

switch between the INTRC and the primary system

clock after 1024 clocks (OST) and 8 clocks of the

primary oscillator. This is conditional upon the SCS

bits being set equal to ‘00’.

• SCS bits are modified from their original value.

• IRCF bits are modified from their original value.

Note:

The instruction to immediately follow the

modification of SCS<1:0> will have an

instruction time (TCY) based on the previ-

ous clock source. This should be taken

into consideration when developing time

dependant code.

Note:

Because the SCS bits are cleared on any

Reset, no clock switching will occur on a

Reset unless the Two-Speed Start-up is

enabled and the primary clock is XT, HS or

LP. The device will wait for the primary

clock to become stable before execution

begins (Two-Speed Start-up disabled).

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