70592C datasheet(135/314 Pages) MICROCHIP

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DS70592C-page 135

PIC24HJXXXGPX06A/X08A/X10A

9.2

Clock Switching Operation

Applications are free to switch between any of the four

clock sources (Primary, LP, FRC and LPRC) under

software control at any time. To limit the possible side

effects

that

could

result

from

this

flexibility,

PIC24HJXXXGPX06A/X08A/X10A devices have a

safeguard lock built into the switch process.

9.2.1

ENABLING CLOCK SWITCHING

To enable clock switching, the FCKSM1 Configuration

bit in the Configuration register must be programmed to

‘0’. (Refer to Section 21.1 “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.

9.2.2

OSCILLATOR SWITCHING SEQUENCE

At a minimum, performing a clock switch requires this

basic sequence:

desired,

read

the

COSC

bits

(OSCCON<14:12>) to determine the current

oscillator source.

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.

Once the basic sequence is completed, the system

clock hardware responds automatically as follows:

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

case, the OSWEN bit is cleared automatically

and the clock switch is aborted.

If a valid clock switch has been initiated, the

LOCK

(OSCCON<5>)

and

the

(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

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).

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).

9.3

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.

If an oscillator failure occurs, 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> Config-

uration bits. While an application can

switch to and from Primary Oscillator

mode in software, it cannot switch

between 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.

2: Direct clock switches between any pri-

mary oscillator mode with PLL and

FRCPLL mode are not permitted. This

applies to clock switches in either direc-

tion. In these instances, the application

must switch to FRC mode as a transition

clock source between the two PLL modes.

3: Refer

Section

“Oscillator”

(DS70227) in the “dsPIC33F/PIC24H

Family Reference Manual” for details.

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