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PIC16(L)F1508/9
DS40001609E-page 58
2011-2015 Microchip Technology Inc.
SCS<1:0> =
00
:
Change the SCS bits in the OSCCON register
to select the FOSC Configuration Word clock
selection. Since the OST is not applicable with
external clocks, the clock module will
immediately switch to the external clock, and
the fail-safe condition will be cleared.
If the Fail-Safe condition still exists, the OSFIF bit will
again be set by hardware.
5.5.3.3
Secondary Oscillator with
SCS<1:0> =
01
When a Fail-Safe condition occurs with the clock switch
selected to run from the Secondary Oscillator selection
(SCS<1:0> =
01
), regardless of the FOSC selection,
the condition is cleared by performing the following pro-
cedure.
SCS<1:0> =
01
(Secondary Oscillator)
SCS<1:0> =
1x
:
Change the SCS bits in the OSCCON register
to select the internal oscillator block.
OSFIF =
0
:
Clear the OSFIF bit in the PIR2 register.
Read SOSCR:
The OST is not used with the secondary
oscillator, therefore, the user must determine if
the secondary oscillator is ready by monitoring
the SOSCR bit in the OSCSTAT register.
When the SOSCR bit is set, the secondary
oscillator is ready.
SCS<1:0> =
01
:
Change the SCS bits in the OSCCON register
to select the secondary oscillator. The clock
module will immediately switch to the
secondary oscillator and the fail-safe condition
will be cleared.
If the Fail-Safe condition still exists, the OSFIF bit will
again be set by hardware.
5.5.4
RESET OR WAKE-UP FROM SLEEP
The FSCM is designed to detect external oscillator or
external clock failures.
When FSCM is used with an external oscillator, the
Oscillator Start-up Timer (OST) count must expire
before the FSCM becomes active. The OST is used
after waking up from Sleep and after any type of Reset.
When the FSCM is used with external clocks, the OST
is not used and the FSCM will be active as soon as the
Reset or wake-up has completed.
When the FSCM is enabled, the Two-Speed Start-up is
also enabled. Therefore, the device will always be exe-
cuting code while the OST is operating.
FIGURE 5-10:
FSCM TIMING DIAGRAM
Note:
Due to the wide range of oscillator start-up
times, the Fail-Safe circuit is not active
during oscillator start-up (i.e., after exiting
Reset or Sleep).
OSFIF
System
Clock
Output
Sample Clock
Failure
Detected
Oscillator
Failure
Note:
The system clock is normally at a much higher frequency than the sample clock. The relative frequencies in
this example have been chosen for clarity.
(Q)
Test
Test
Test
Clock Monitor Output
Содержание PIC12F1501
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