PIC18F87K22 Datasheet, PDF (56/548 Pages) Microchip Technology – 64/80-Pin, High-Performance, 1-Mbit Enhanced Flash Microcontrollers with 12-Bit A/D and nanoWatt XLP Technology

PIC18F87K22 FAMILY
4.1.3
CLOCK TRANSITIONS AND STATUS
INDICATORS
The length of the transition between clock sources is
the sum of two cycles of the old clock source and three
to four cycles of the new clock source. This formula
assumes that the new clock source is stable. The HF-
INTOSC and MF-INTOSC are termed as INTOSC in
this chapter.
Three bits indicate the current clock source and its
status, as shown in Table 4-2. The three bits are:
• OSTS (OSCCON<3>)
• HFIOFS (OSCCON<2>)
• SOSCRUN (OSCCON2<6>)
TABLE 4-2: SYSTEM CLOCK INDICATOR
Main Clock Source
OSTS
HFIOFS or
MFIOFS
SOSCRUN
Primary oscillator
1
0
0
INTOSC (HF-INTOSC
or MF-INTOSC)
0
1
0
Secondary Oscillator
0
0
1
MF-INTOSC or
HF-INTOSC as primary 1
1
0
clock source
LF-INTOSC is running
or INTOSC is not yet
0
0
0
stable
When the OSTS bit is set, the primary clock is providing
the device clock. When the HFIOFS or MFIOFS bit is
set, the INTOSC output is providing a stable 16 MHz
clock source to a divider that actually drives the device
clock. When the SOSCRUN bit is set, the SOSC oscil-
lator is providing the clock. If none of these bits are set,
either the LF-INTOSC clock source is clocking the
device or the INTOSC source is not yet stable.
If the internal oscillator block is configured as the
primary clock source by the FOSC<3:0> Configuration
bits (CONFIG1H<3:0>), then the OSTS and HFIOFS or
MFIOFS bits can be set when in PRI_RUN or
PRI_IDLE modes. This indicates that the primary clock
(INTOSC output) is generating a stable 16 MHz output.
Entering another INTOSC power-managed mode at
the same frequency would clear the OSTS bit.
Note 1: Caution should be used when modifying a
single IRCF bit. At a lower VDD, it is
possible to select a higher clock speed
than is supportable by that VDD. Improper
device operation may result if the VDD/
FOSC specifications are violated.
2: Executing a SLEEP instruction does not
necessarily place the device into Sleep
mode. It acts as the trigger to place the
controller into either the Sleep mode or
one of the Idle modes, depending on the
setting of the IDLEN bit.
4.1.4 MULTIPLE SLEEP COMMANDS
The power-managed mode that is invoked with the
SLEEP instruction is determined by the setting of the
IDLEN bit at the time the instruction is executed. If
another SLEEP instruction is executed, the device will
enter the power-managed mode specified by IDLEN at
that time. If IDLEN has changed, the device will enter
the new power-managed mode specified by the new
setting.
4.2 Run Modes
In the Run modes, clocks to both the core and
peripherals are active. The difference between these
modes is the clock source.
4.2.1 PRI_RUN MODE
The PRI_RUN mode is the normal, full-power execu-
tion mode of the microcontroller. This is also the default
mode upon a device Reset, unless Two-Speed Start-up
is enabled. (For details, see Section 28.4 “Two-Speed
Start-up”.) In this mode, the OSTS bit is set. The
HFIOFS or MFIOFS bit may be set if the internal
oscillator block is the primary clock source. (See
Section 3.2 “Control Registers”.)
4.2.2 SEC_RUN MODE
The SEC_RUN mode is the compatible mode to the
“clock-switching” feature offered in other PIC18
devices. In this mode, the CPU and peripherals are
clocked from the SOSC oscillator. This enables lower
power consumption while retaining a high-accuracy
clock source.
SEC_RUN mode is entered by setting the SCS<1:0>
bits to ‘01’. The device clock source is switched to the
SOSC oscillator (see Figure 4-1), the primary oscillator
is shut down, the SOSCRUN bit (OSCCON2<6>) is set
and the OSTS bit is cleared.
Note:
The SOSC oscillator can be enabled by
setting the SOSCGO bit (OSCCON2<3>).
If this bit is set, the clock switch to the
SEC_RUN mode can switch immediately
once SCS<1:0> are set to ‘01’.
On transitions from SEC_RUN mode to PRI_RUN
mode, the peripherals and CPU continue to be clocked
from the SOSC oscillator while the primary clock is
started. When the primary clock becomes ready, a
clock switch back to the primary clock occurs (see
Figure 4-2). When the clock switch is complete, the
SOSCRUN bit is cleared, the OSTS bit is set and the
primary clock is providing the clock. The IDLEN and
SCS bits are not affected by the wake-up and the
SOSC oscillator continues to run.
DS39960B-page 56
Preliminary
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