PIC16F87-E Datasheet, PDF (41/228 Pages) Microchip Technology – 18/20/28-Pin Enhan lashMicrocontrollers with nanoWatt Technology

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
register details.
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
in 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
 2005 Microchip Technology Inc.
PIC16F87/88
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’.
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.
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:
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).
DS30487C-page 39