Microchip Technology MA330016 Data Sheet
© 2007-2011 Microchip Technology Inc.
DS70290J-page 107
dsPIC33FJ32GP202/204 and dsPIC33FJ16GP304
8.4
Clock Switching Operation
Applications are free to switch among any of the four
clock sources (Primary, LP, FRC and LPRC) under
software control at any time. To limit the possible side
effects of this flexibility, dsPIC33FJ32GP202/204 and
dsPIC33FJ16GP304 devices have a safeguard lock
built into the switch process.
clock sources (Primary, LP, FRC and LPRC) under
software control at any time. To limit the possible side
effects of this flexibility, dsPIC33FJ32GP202/204 and
dsPIC33FJ16GP304 devices have a safeguard lock
built into the switch process.
8.4.1
ENABLING CLOCK SWITCHING
To enable clock switching, the FCKSM1 Configuration
bit in the Configuration register must be programmed to
‘0’. (Refer to
bit in the Configuration register must be programmed to
‘0’. (Refer to
further details.) If the FCKSM1 Configuration bit is
unprogrammed (‘1’), the clock switching function and
Fail-Safe Clock Monitor function are disabled. This is
the default setting.
The NOSC control bits (OSCCON<10:8>) do not
control the clock selection when clock switching is
disabled. However, the COSC bits (OSCCON<14:12>)
reflect the clock source selected by the FNOSC
Configuration bits.
The OSWEN control bit (OSCCON<0>) has no effect
when clock switching is disabled. It is held at ‘0’ at all
times.
unprogrammed (‘1’), the clock switching function and
Fail-Safe Clock Monitor function are disabled. This is
the default setting.
The NOSC control bits (OSCCON<10:8>) do not
control the clock selection when clock switching is
disabled. However, the COSC bits (OSCCON<14:12>)
reflect the clock source selected by the FNOSC
Configuration bits.
The OSWEN control bit (OSCCON<0>) has no effect
when clock switching is disabled. It is held at ‘0’ at all
times.
8.4.2
OSCILLATOR SWITCHING
SEQUENCE
SEQUENCE
Performing a clock switch requires this basic
sequence:
1.
sequence:
1.
If desired, read the COSC bits
(OSCCON<14:12>) to determine the current
oscillator source.
(OSCCON<14:12>) to determine the current
oscillator source.
2.
Perform the unlock sequence to allow a write to
the OSCCON register high byte.
the OSCCON register high byte.
3.
Write the appropriate value to the NOSC control
bits (OSCCON<10:8>) for the new oscillator
source.
bits (OSCCON<10:8>) for the new oscillator
source.
4.
Perform the unlock sequence to allow a write to
the OSCCON register low byte.
the OSCCON register low byte.
5.
Set the OSWEN bit to initiate the oscillator
switch.
switch.
Once the basic sequence is completed, the system
clock hardware responds automatically as follows:
1.
clock hardware responds automatically as follows:
1.
The clock switching hardware compares the
COSC status bits with the new value of the
NOSC control bits. If they are the same, the
clock switch is a redundant operation. In this
COSC status bits with the new value of the
NOSC control bits. If they are the same, the
clock switch is a redundant operation. In this
case, the OSWEN bit is cleared automatically
and the clock switch is aborted.
and the clock switch is aborted.
2.
If a valid clock switch has been initiated, the
status bits, LOCK (OSCCON<5>) and CF
(OSCCON<3>) are cleared.
status bits, LOCK (OSCCON<5>) and CF
(OSCCON<3>) are cleared.
3.
The new oscillator is turned on by the hardware
if it is not currently running. If a crystal oscillator
must be turned on, the hardware waits until the
Oscillator Start-up Timer (OST) expires. If the
new source is using the PLL, the hardware waits
until a PLL lock is detected (LOCK = 1).
if it is not currently running. If a crystal oscillator
must be turned on, the hardware waits until the
Oscillator Start-up Timer (OST) expires. If the
new source is using the PLL, the hardware waits
until a PLL lock is detected (LOCK = 1).
4.
The hardware waits for 10 clock cycles from the
new clock source and then performs the clock
switch.
new clock source and then performs the clock
switch.
5.
The hardware clears the OSWEN bit to indicate a
successful clock transition. In addition, the NOSC
bit values are transferred to the COSC status bits.
successful clock transition. In addition, the NOSC
bit values are transferred to the COSC status bits.
6.
The old clock source is turned off at this time,
with the exception of LPRC (if WDT or FSCM
are enabled) or LP (if LPOSCEN remains set).
with the exception of LPRC (if WDT or FSCM
are enabled) or LP (if LPOSCEN remains set).
8.5
Fail-Safe Clock Monitor (FSCM)
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 programming.
If the FSCM function is enabled, the LPRC internal
oscillator runs at all times (except during Sleep mode)
and is not subject to control by the Watchdog Timer.
In the event of an oscillator failure, the FSCM
generates a clock failure trap event and switches the
system clock over to the FRC oscillator. Then the
application program can either attempt to restart the
oscillator or execute a controlled shutdown. The trap
can be treated as a warm Reset by simply loading the
Reset address into the oscillator fail trap vector.
If the PLL multiplier is used to scale the system clock,
the internal FRC is also multiplied by the same factor
on clock failure. Essentially, the device switches to
FRC with PLL on a clock failure.
to continue to operate even in the event of an oscillator
failure. The FSCM function is enabled by programming.
If the FSCM function is enabled, the LPRC internal
oscillator runs at all times (except during Sleep mode)
and is not subject to control by the Watchdog Timer.
In the event of an oscillator failure, the FSCM
generates a clock failure trap event and switches the
system clock over to the FRC oscillator. Then the
application program can either attempt to restart the
oscillator or execute a controlled shutdown. The trap
can be treated as a warm Reset by simply loading the
Reset address into the oscillator fail trap vector.
If the PLL multiplier is used to scale the system clock,
the internal FRC is also multiplied by the same factor
on clock failure. Essentially, the device switches to
FRC with PLL on a clock failure.
Note:
Primary Oscillator mode has three different
submodes (XT, HS and EC), which are
determined by the POSCMD<1:0>
Configuration bits. While an application
can switch to and from Primary Oscillator
mode in software, it cannot switch among
the different primary submodes without
reprogramming the device.
submodes (XT, HS and EC), which are
determined by the POSCMD<1:0>
Configuration bits. While an application
can switch to and from Primary Oscillator
mode in software, it cannot switch among
the different primary submodes without
reprogramming the device.
Note 1: The processor continues to execute code
throughout the clock switching sequence.
Timing-sensitive code should not be
executed during this time.
Timing-sensitive code should not be
executed during this time.
2: Direct clock switches between any pri-
mary oscillator mode with PLL and FRC-
PLL mode are not permitted. This applies
to clock switches in either direction. In
these instances, the application must
switch to FRC mode as a transition clock
source between the two PLL modes.
PLL mode are not permitted. This applies
to clock switches in either direction. In
these instances, the application must
switch to FRC mode as a transition clock
source between the two PLL modes.
3: Refer to Section 7. “Oscillator”
(DS70186) in the “dsPIC33F/PIC24H
Family Reference Manual” for details.
Family Reference Manual” for details.