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2010 Microchip Technology Inc.

Preliminary

DS41350E-page 23

PIC18F/LF1XK50

2.7

Oscillator Start-up Timer

The Primary External Oscillator, when configured for

LP, XT or HS modes, incorporates an Oscillator Start-up

Timer (OST). The OST ensures that the oscillator starts

and provides a stable clock to the oscillator module.

The OST times out when 1024 oscillations on OSC1

have occurred. During the OST period, with the system

clock set to the Primary External Oscillator, the program

counter does not increment suspending program

execution. The OST period will occur following:

Power-on Reset (POR)

Brown-out Reset (BOR)

Wake-up from Sleep

Oscillator being enabled

Expiration of Power-up Timer (PWRT)

In order to minimize latency between external oscillator

start-up and code execution, the Two-Speed Start-up

mode can be selected. See Section 2.12 “Two-Speed

Start-up Mode” for more information.

2.8

Clock Switching

The device contains circuitry to prevent clock “glitches”

due to a change of the system clock source. To

accomplish this, a short pause in the system clock

occurs during the clock switch. If the new clock source

is not stable (e.g., OST is active), the device will

continue to execute from the old clock source until the

new clock source becomes stable. The timing of a

clock switch is as follows:

SCS<1:0> bits of the OSCCON register are

modified.

The system clock will continue to operate from

the old clock until the new clock is ready.

Clock switch circuitry waits for two consecutive

rising edges of the old clock after the new clock

is ready.

The system clock is held low, starting at the next

falling edge of the old clock.

Clock switch circuitry waits for an additional two

rising edges of the new clock.

On the next falling edge of the new clock, the

low hold on the system clock is release and the

new clock is switched in as the system clock.

Clock switch is complete.

Refer to Figure 2-5 for more details.

FIGURE 2-5:

CLOCK SWITCH TIMING

Old Clock

New Clock

IRCF <2:0>

System Clock

Start-up Time(1)

Clock Sync

Running

High Speed

Low Speed

Select Old

Select New

New Clk Ready

Low Speed

High Speed

Old Clock

New Clock

IRCF <2:0>

System Clock

Start-up Time(1)

Clock Sync

Running

Select Old

Select New

New Clk Ready

Note 1: Start-up time includes TOST (1024 TOSC) for external clocks, plus TPLL (approx. 2 ms) for HSPLL mode.

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