PIC18F45J11-I/ML【PDF 61/228 页】IC PIC MCU FLASH 32K 2V 44-QFN

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Sheet目录3862

57页 58页 59页 60页 [61页]62页 63页 64页 65页

2010 Microchip Technology Inc.

DS70138G-page 153

dsPIC30F3014/4013

20.4.1.1

POR with Long Crystal Start-up Time

(with FSCM Enabled)

The oscillator start-up circuitry is not linked to the POR

circuitry.

Some

crystal

circuits

(especially

low-

frequency crystals) have a relatively long start-up time.

Therefore, one or more of the following conditions is

possible after the POR timer and the PWRT have

expired:

The oscillator circuit has not begun to oscillate.

The Oscillator Start-up Timer has not expired (if a

crystal oscillator is used).

The PLL has not achieved a LOCK (if PLL is

used).

If the FSCM is enabled and one of the above conditions

is true, a clock failure trap occurs. The device automat-

ically switches to the FRC oscillator and the user can

switch to the desired crystal oscillator in the trap ISR.

20.4.1.2

Operating without FSCM and PWRT

If the FSCM is disabled and the Power-up Timer

(PWRT) is also disabled, then the device exits rapidly

from Reset on power-up. If the clock source is FRC,

LPRC, ERC or EC, it will be active immediately.

If the FSCM is disabled and the system clock has not

started, the device will be in a frozen state at the Reset

vector until the system clock starts. From the user’s

perspective, the device appears to be in Reset until a

system clock is available.

20.4.2

BOR: PROGRAMMABLE

BROWN-OUT RESET

The BOR (Brown-out Reset) module is based on an

internal voltage reference circuit. The main purpose of

the BOR module is to generate a device Reset when a

brown-out condition occurs. Brown-out conditions are

generally caused by glitches on the AC mains (i.e.,

missing portions of the AC cycle waveform due to bad

power transmission lines, or voltage sags due to exces-

sive current draw when a large inductive load is turned

on).

The BOR module allows selection of one of the

following voltage trip points (see Table 23-11):

2.6V-2.71V

4.1V-4.4V

4.58V-4.73V

A BOR generates a Reset pulse, which resets the

device. The BOR selects the clock source based on the

device Configuration

bit values

(FOS<2:0> and

FPR<4:0>). Furthermore, if an oscillator mode is

selected, the BOR activates the Oscillator Start-up

Timer (OST). The system clock is held until OST

expires. If the PLL is used, then the clock is held until

the LOCK bit (OSCCON<5>) is ‘1’.

Concurrently, the POR time-out (TPOR) and the PWRT

time-out (TPWRT) are applied before the internal Reset is

released. If TPWRT = 0 and a crystal oscillator is being

used, then a nominal delay of TFSCM = 100

s is applied.

The total delay in this case is (TPOR + TFSCM).

The BOR status bit (RCON<1>) is set to indicate that a

BOR has occurred. The BOR circuit, if enabled, contin-

ues to operate while in Sleep or Idle modes and resets

the device should VDD fall below the BOR threshold

voltage.

FIGURE 20-6:

EXTERNAL POWER-ON

RESET CIRCUIT (FOR

SLOW VDD POWER-UP)

Note:

The BOR voltage trip points indicated here

are nominal values provided for design

guidance only. Refer to the Electrical

Specifications in the specific device data

sheet for BOR voltage limit specifications.

Note:

Dedicated supervisory devices, such as

the MCP1XX and MCP8XX, may also be

used as an external Power-on Reset

circuit.

Note

External Power-on Reset circuit is required

only if the VDD power-up slope is too slow.

The diode D helps discharge the capacitor

quickly when VDD powers down.

R should be suitably chosen so as to make

sure that the voltage drop across R does not

violate the device’s electrical specifications.

R1 should be suitably chosen so as to limit

any current flowing into MCLR from external

capacitor C, in the event of MCLR/VPP pin

breakdown due to Electrostatic Discharge

(ESD), or Electrical Overstress (EOS).

VDD

dsPIC30F

MCLR

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