Microchip Technology dsPIC33F Family Manual do Utilizador descarregar pdf (Página 22)

dsPIC33F

FIGURE 6-1: OSCILLATOR SYSTEM BLOCK DIAGRAM

6.2 Power-on Reset

When a supply voltage is applied to the device, a

Power-on Reset is generated. A new Power-on Reset

event is generated if the supply voltage falls below the

device threshold voltage (V

POR). An internal POR

pulse is generated when the rising supply voltage

crosses the POR circuit threshold voltage.

6.3 Oscillator Start-up Timer/Stabilizer

(OST)

An Oscillator Start-up Timer (OST) is included to

ensure that a crystal oscillator (or ceramic resonator)

has started and stabilized. The OST is a simple, 10-bit

counter that counts 1024 T

OSC cycles before releasing

the oscillator clock to the rest of the system. The time-

out period is designated as T

OST. The TOST time is

involved every time the oscillator has to restart (i.e., on

Power-on Reset (POR) and wake-up from Sleep). The

Oscillator Start-up Timer is applied to the LP oscillator,

XT and HS modes (upon wake-up from Sleep, POR

and BOR) for the primary oscillator.

6.4 Watchdog Timer (WDT)

The primary function of the Watchdog Timer (WDT) is

to reset the processor in the event of a software

malfunction. The WDT is a free-running timer that runs

off the on-chip LPRC oscillator, requiring no external

component. The WDT continues to operate even if the

main processor clock (e.g., the crystal oscillator) fails.

The Watchdog Timer can be “Enabled” or “Disabled”

either through a configuration bit (FWDTEN) in the

Configuration register, or through an SFR bit

(SWDTEN).

Any device programmer capable of programming

dsPIC

DSC devices (such as Microchip’s MPLAB

PM3 Programmer) allows programming of this and

other configuration bits to the desired state. If enabled,

the WDT increments until it overflows or “times out”. A

WDT time-out forces a device Reset (except during

Sleep).

OSC1

Secondary

Oscillator

32 kHz

PLL

Module

Clock

Switching

and

Control

Block

OSC2

SOSCO

SOSCI

FOSC

Primary Osc

Secondary Osc

To Ti me r1

Internal Fast

RC (FRC)

Oscillator

Internal Low-Power

RC (LPRC)

Oscillator

Primary

Oscillator

FCY

Divide by 4

1 2 ... 17 18 19 20 21 22 23 24 25 26 27 ... 89 90

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