參數(shù)資料
型號: DV164136
廠商: Microchip Technology
文件頁數(shù): 43/107頁
文件大小: 0K
描述: DEVELOPMENT KIT FOR PIC18
產品培訓模塊: PIC18 J Series MCU Overview
標準包裝: 1
系列: PIC®
類型: MCU
適用于相關產品: PIC18F8722,PIC18F87J11
所含物品: 板,線纜,CD,PICkit? 3 個編程器,電源
產品目錄頁面: 659 (CN2011-ZH PDF)
相關產品: PIC18F87J11-I/PT-ND - IC PIC MCU FLASH 64KX16 80TQFP
PIC18F87J11T-I/PTTR-ND - IC PIC MCU FLASH 64KX16 80TQFP
PIC18F8722T-E/PT-ND - IC PIC MCU FLASH 64KX16 80TQFP
PIC18F8722-E/PT-ND - IC PIC MCU FLASH 64KX16 80TQFP
PIC18F8722T-I/PT-ND - IC PIC MCU FLASH 64KX16 80TQFP
PIC18F8722-I/PT-ND - IC PIC MCU FLASH 64KX16 80TQFP
PIC18F87J11 FAMILY
DS39778E-page 40
2007-2012 Microchip Technology Inc.
3.3.1
CLOCK SOURCE SELECTION
The
System
Clock
Select
bits,
SCS<1:0>
(OSCCON<1:0>), select the clock source. The avail-
able clock sources are the primary clock defined by the
FOSC<2:0> Configuration bits, the secondary clock
(Timer1 oscillator) and the internal oscillator. The clock
source changes after one or more of the bits are written
to, following a brief clock transition interval.
The OSTS (OSCCON<3>) and T1RUN (T1CON<6>)
bits indicate which clock source is currently providing
the device clock. The OSTS bit indicates that the
Oscillator Start-up Timer (OST) has timed out and the
primary clock is providing the device clock in primary
clock modes. The T1RUN bit indicates when the
Timer1 oscillator is providing the device clock in sec-
ondary clock modes. In power-managed modes, only
one of these bits will be set at any time. If neither of
these bits is set, the INTRC is providing the clock, or
the internal oscillator has just started and is not yet
stable.
The IDLEN bit determines if the device goes into Sleep
mode or one of the Idle modes when the SLEEP
instruction is executed.
The use of the flag and control bits in the OSCCON
register is discussed in more detail in Section 4.0
3.3.1.1
System Clock Selection and Device
Resets
Since the SCSx bits are cleared on all forms of Reset,
this means the primary oscillator defined by the
FOSC<2:0> Configuration bits is used as the primary
clock source on device Resets. This could either be the
internal oscillator block by itself, or one of the other
primary clock sources (HS, EC, HSPLL, ECPLL1/2 or
INTPLL1/2).
In those cases when the internal oscillator block, with-
out PLL, is the default clock on Reset, the Fast RC
Oscillator (INTOSC) will be used as the device clock
source. It will initially start at 4 MHz; the postscaler
selection that corresponds to the Reset value of the
IRCF<2:0> bits (‘110’).
Regardless of which primary oscillator is selected,
INTRC will always be enabled on device power-up. It
serves as the clock source until the device has loaded
its configuration values from memory. It is at this point
that the FOSCx Configuration bits are read and the
oscillator selection of the operational mode is made.
Note that either the primary clock source, or the internal
oscillator, will have two bit setting options for the possible
values of the SCS<1:0> bits at any given time.
3.3.2
OSCILLATOR TRANSITIONS
PIC18F87J11 family devices contain circuitry to
prevent 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
.
Note 1:
The Timer1 oscillator must be enabled to
select the secondary clock source. The
Timer1 oscillator is enabled by setting the
T1OSCEN bit in the Timer1 Control regis-
ter (T1CON<3>). If the Timer1 oscillator is
not enabled, then any attempt to select a
secondary clock source when executing a
SLEEP
instruction will be ignored.
2:
It is recommended that the Timer1
oscillator be operating and stable before
executing the SLEEP instruction or a very
long delay may occur while the Timer1
oscillator starts.
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