DSPIC30F MICROCHIP [Microchip Technology], DSPIC30F Datasheet - Page 160

• Manufacturer Part Number: DSPIC30F
• Description: General Purpose and Sensor Families High-Performance Digital Signal Controllers
• Manufacturer: MICROCHIP [Microchip Technology]

dsPIC30F
20.2.6
The LPRC oscillator is a component of the Watchdog
Timer (WDT) and oscillates at a nominal frequency of
512 kHz. The LPRC oscillator is the clock source for
the Power-up Timer (PWRT) circuit, WDT, and clock
monitor circuits. It may also be used to provide a low
frequency clock source option for applications where
power consumption is critical and timing accuracy is
not required
The LPRC oscillator is always enabled at a Power-on
Reset because it is the clock source for the PWRT.
After the PWRT expires, the LPRC oscillator will
remain on if one of the following is TRUE:
• The Fail-Safe Clock Monitor is enabled
• The WDT is enabled
• The LPRC oscillator is selected as the system
If one of the above conditions is not true, the LPRC will
shut-off after the PWRT expires.
20.2.7
The Fail-Safe Clock Monitor (FSCM) allows the device
to continue to operate even in the event of an oscillator
failure. The FSCM function is enabled by appropriately
programming the FCKSM configuration bits (clock
switch and monitor selection bits) in the FOSC Device
Configuration register. If the FSCM function is enabled,
the LPRC internal oscillator will run at all times (except
during Sleep mode) and will not be subject to control by
the SWDTEN bit.
In the event of an oscillator failure, the FSCM will gen-
erate a clock failure trap event and will switch the sys-
tem clock over to the FRC oscillator. The user will then
have the option to either attempt to restart the oscillator
or execute a controlled shutdown. The user may decide
to treat the trap as a warm Reset by simply loading the
Reset address into the oscillator fail trap vector. In this
event, the CF (Clock Fail) status bit (OSCCON<3>) is
also set whenever a clock failure is recognized.
In the event of a clock failure, the WDT is unaffected
and continues to run on the LPRC clock.
DS70083G-page 158
clock via the COSC<2:0> control bits in the
OSCCON register
Note 1: OSC2 pin function is determined by the
2: OSC1 pin cannot be used as an I/O pin
LOW POWER RC OSCILLATOR
(LPRC)
FAIL-SAFE CLOCK MONITOR
Primary
(FPR<4:0>).
even if the secondary oscillator or an
internal clock source is selected at all
times.
Oscillator
mode
selection
Preliminary
If the oscillator has a very slow start-up time coming out
of POR, BOR or Sleep, it is possible that the PWRT
timer will expire before the oscillator has started. In
such cases, the FSCM will be activated and the FSCM
will initiate a clock failure trap, and the COSC<2:0> bits
are loaded with FRC oscillator selection. This will effec-
tively shut-off the original oscillator that was trying to
start.
The user may detect this situation and restart the
oscillator in the clock fail trap ISR.
Upon a clock failure detection, the FSCM module will
initiate a clock switch to the FRC oscillator as follows:
1.
2.
3.
For the purpose of clock switching, the clock sources
are sectioned into four groups:
1.
2.
3.
4.
The user can switch between these functional groups
but cannot switch between options within a group. If the
primary group is selected, then the choice within the
group is always determined by the FPR<4:0>
configuration bits.
The OSCCON register holds the control and status bits
related to clock switching.
• COSC<1:0>: Read only status bits always reflect
• NOSC<2:0>: Control bits which are written to
• LOCK: The LOCK status bit indicates a PLL lock.
• CF: Read only status bit indicating if a clock fail
• OSWEN: Control bit changes from a ‘0’ to a ‘1’
If configuration bits FCKSM<2:0> = 1x, then the clock
switching and Fail-Safe Clock monitoring functions are
disabled. This is the default configuration bit setting.
the current oscillator group in effect.
indicate the new oscillator group of choice.
- On POR and BOR, COSC<2:0> and
detect has occurred.
when a clock transition sequence is initiated.
Clearing the OSWEN control bit will abort a clock
transition in progress (used for hang-up
situations).
The COSC bits (OSCCON<14:12>) are loaded
with the FRC oscillator selection value.
CF bit is set (OSCCON<3>).
OSWEN control bit (OSCCON<0>) is cleared.
Primary
Secondary
Internal FRC
Internal LPRC
NOSC<1:0> are both loaded with the
configuration bit values FOS<2:0>.
 2004 Microchip Technology Inc.