PIC18F23K20-ESLASHML datasheet(37/456 Pages) MICROCHIP

37 / 456 page

 2010 Microchip Technology Inc.

DS41303G-page 37

PIC18F2XK20/4XK20

2.9

Clock Switching

The system clock source can be switched between

external and internal clock sources via software using

the System Clock Select (SCS<1:0>) bits of the

OSCCON register.

PIC18F2XK20/4XK20 devices contain circuitry to pre-

vent clock “glitches” when switching between clock

sources. A short pause in the device clock occurs dur-

ing the clock switch. The length of this pause is the sum

of two cycles of the old clock source and three to four

cycles of the new clock source. This formula assumes

that the new clock source is stable.

Clock transitions are discussed in greater detail in

Section 3.1.2 “Entering Power-Managed Modes”.

2.9.1

SYSTEM CLOCK SELECT

(SCS<1:0>) BITS

The System Clock Select (SCS<1:0>) bits of the

OSCCON register select the system clock source that

is used for the CPU and peripherals.

• When SCS<1:0> = 00, the system clock source is

determined by configuration of the FOSC<2:0>

bits in the CONFIG1H Configuration register.

• When SCS<1:0> = 10, the system clock source is

chosen by the internal oscillator frequency

selected by the INTSRC bit of the OSCTUNE

• When SCS<1:0> = 01, the system clock source is

the 32.768 kHz secondary oscillator shared with

Timer1.

After a Reset, the SCS<1:0> bits of the OSCCON

2.9.2

OSCILLATOR START-UP TIME-OUT

STATUS (OSTS) BIT

The Oscillator Start-up Time-out Status (OSTS) bit of

the OSCCON register indicates whether the system

clock is running from the external clock source, as

defined by the FOSC<3:0> bits in the CONFIG1H

Configuration register, or from the internal clock

source. In particular, when the primary oscillator is the

source of the primary clock, OSTS indicates that the

Oscillator Start-up Timer (OST) has timed out for LP,

XT or HS modes.

2.9.3

CLOCK SWITCH TIMING

When switching between one oscillator and another,

the new oscillator may not be operating which saves

power (see Figure 2-7). If this is the case, there is a

delay after the SCS<1:0> bits of the OSCCON register

are modified before the frequency change takes place.

The OSTS and IOFS bits of the OSCCON register will

reflect the current active status of the external and

HFINTOSC oscillators. The timing of a frequency

selection is as follows:

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

ified.

The old clock continues to operate until the new

clock is ready.

Clock switch circuitry waits for two consecutive

rising edges of the old clock after the new clock

ready signal goes true.

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 released and new

clock is switched in as the system clock.

Clock switch is complete.

See Figure 2-1 for more details.

If the HFINTOSC is the source of both the old and new

frequency, there is no start-up delay before the new

frequency is active. This is because the old and new

frequencies are derived from the HFINTOSC via the

postscaler and multiplexer.

Start-up

delay

specifications

are

located

Section 26.0 “Electrical Characteristics”, under AC

Specifications (Oscillator Module).

Note:

Any automatic clock switch, which may

occur from Two-Speed Start-up or Fail-Safe

Clock

Monitor,

does

not

update

the

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

The user can monitor the T1RUN bit of the

T1CON register and the IOFS and OSTS

bits of the OSCCON register to determine

the current system clock source.

PIC18F23K20-E/ML датащи(PDF) 37 Page - Microchip Technology

PIC18F23K20-E/ML датащи(HTML) 37 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

датащи скачать

ссылки URL