pic24hj16gp304 Microchip Technology Inc., pic24hj16gp304 Datasheet - Page 91

• Manufacturer Part Number: pic24hj16gp304
• Description: High-performance, 16-bit Microcontrollers
• Manufacturer: Microchip Technology Inc.

7.2
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, PIC24HJ32GP202/204 and
PIC24HJ16GP304 devices have a safeguard lock built
into the switch process.
7.2.1
To enable clock switching, the FCKSM1 Configuration
bit in the Configuration register must be programmed to
‘0’. (Refer to Section 18.1 “Configuration Bits” for
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.
7.2.2
Performing a clock switch requires the following basic
sequence:
1.
2.
3.
4.
5.
Once the basic sequence is completed, the system
clock hardware responds automatically as follows:
1.
© 2007 Microchip Technology Inc.
Note:
Read the COSC bits (OSCCON<14:12>) to
determine the current oscillator source, if
desired.
Perform the unlock sequence to allow a write to
the OSCCON register high byte.
Write the appropriate value to the NOSC control
bits (OSCCON<10:8>) for the new oscillator
source.
Perform the unlock sequence to allow a write to
the OSCCON register low byte.
Set the OSWEN bit to initiate the oscillator
switch.
The clock switching hardware compares the
COSC status bits with the new value of the
NOSC control bits. If both of them are the same,
the clock switch is a redundant operation. In this
Clock Switching Operation
Primary Oscillator mode has three different
submodes (XT, HS and EC), which are
determined by the POSCMD<1:0> Config-
uration 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.
ENABLING CLOCK SWITCHING
OSCILLATOR SWITCHING
SEQUENCE
PIC24HJ32GP202/204 and PIC24HJ16GP304
Preliminary
2.
3.
4.
5.
6.
7.3
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.
Note 1: The processor continues to execute code
case, the OSWEN bit is cleared automatically
and the clock switch is aborted.
If a valid clock switch has been initiated, the
LOCK
(OSCCON<3>) status bits are cleared.
The new oscillator is turned on by the hardware
if it is not currently running. If a crystal oscillator
has to 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).
The hardware waits for 10 clock cycles from the
new clock source and then performs the clock
switch.
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.
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).
2: Direct clock switches between any primary
Fail-Safe Clock Monitor (FSCM)
throughout the clock switching sequence.
Timing-sensitive code should not be
executed during this time.
oscillator mode with PLL and FRCPLL
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.
(OSCCON<5>)
and
DS-70289A-page 89
the
CF