41594B datasheet(57/372 Pages) MICROCHIP

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 2011-2012 Microchip Technology Inc.

Preliminary

DS41594B-page 57

PIC12LF1840T48A

5.4

Two-Speed Clock Start-up Mode

Two-Speed Start-up mode provides additional power

savings by minimizing the latency between external

oscillator start-up and code execution. In applications

that make heavy use of the Sleep mode, Two-Speed

Start-up will remove the external oscillator start-up

time from the time spent awake and can reduce the

overall power consumption of the device. This mode

allows the application to wake-up from Sleep, perform

a few instructions using the INTOSC Internal Oscillator

block as the clock source and go back to Sleep without

waiting for the external oscillator to become stable.

Two-Speed Start-up provides benefits when the oscil-

lator module is configured for LP, XT or HS modes.

The Oscillator Start-up Timer (OST) is enabled for

these modes and must count 1024 oscillations before

the oscillator can be used as the system clock source.

If the oscillator module is configured for any mode

other than LP, XT or HS mode, then Two-Speed

Start-up is disabled. This is because the external clock

oscillator does not require any stabilization time after

POR or an exit from Sleep.

If the OST count reaches 1024 before the device

enters Sleep mode, the OSTS bit of the OSCSTAT reg-

ister is set and program execution switches to the

external oscillator. However, the system may never

operate from the external oscillator if the time spent

awake is very short.

5.4.1

TWO-SPEED START-UP MODE

CONFIGURATION

Two-Speed Start-up mode is configured by the

following settings:

• IESO (of the Configuration Word 1) = 1; Inter-

nal/External Switchover bit (Two-Speed Start-up

mode enabled).

• SCS (of the OSCCON register) = 00.

• FOSC<2:0> bits in the Configuration Word 1

configured for LP, XT or HS mode.

Two-Speed Start-up mode is entered after:

• Power-on Reset (POR) and, if enabled, after

Power-up Timer (PWRT) has expired, or

• Wake-up from Sleep.

TABLE 5-1:

OSCILLATOR SWITCHING DELAYS

Note:

Executing a SLEEP instruction will abort

the oscillator start-up time and will cause

the OSTS bit of the OSCSTAT register to

remain clear.

Switch From

Switch To

Frequency

Oscillator Delay

Sleep/POR

LFINTOSC(1)

MFINTOSC(1)

HFINTOSC(1)

31 kHz

31.25 kHz-500 kHz

31.25kHz-16MHz

Oscillator Warm-up Delay (TWARM)

Sleep/POR

EC, RC(1)

DC – 32 MHz

2 cycles

LFINTOSC

EC, RC(1)

DC – 32 MHz

1 cycle of each

Sleep/POR

Timer1 Oscillator

LP, XT, HS(1)

32 kHz-20 MHz

1024 Clock Cycles (OST)

Any clock source

MFINTOSC(1)

HFINTOSC(1)

31.25 kHz-500 kHz

31.25kHz-16MHz

s (approx.)

Any clock source

LFINTOSC(1)

31 kHz

1 cycle of each

Any clock source

Timer1 Oscillator

32 kHz

1024 Clock Cycles (OST)

PLL inactive

PLL active

16-32 MHz

2 ms (approx.)

Note 1:

PLL inactive.

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