DSPIC30F3012AT20ESLASHML datasheet(129/205 Pages) MICROCHIP

129 / 205 page

DS70139E-page 127

dsPIC30F2011/2012/3012/3013

17.3.1.1

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 fre-

quency crystals) will 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, then a clock failure trap will occur. The device

will automatically switch to the FRC oscillator and the

user can switch to the desired crystal oscillator in the

trap ISR.

17.3.1.2

Operating without FSCM and PWRT

If the FSCM is disabled and the Power-up Timer

(PWRT) is also disabled, then the device will exit rap-

idly 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 will appear to be in Reset until

a system clock is available.

17.3.2

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 20-11):

• 2.6V-2.71V

•4.1V-4.4V

• 4.58V-4.73V

A BOR will generate a Reset pulse which will reset the

device. The BOR will select 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 will activate the Oscillator Start-up

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

expires. If the PLL is used, then the clock will be held

until the LOCK bit (OSCCON<5>) is ‘1’.

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

time-out (TPWRT) will be applied before the internal Reset

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

used, then a nominal delay of TFSCM = 100

μs is applied.

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

The BOR Status bit (RCON<1>) will be set to indicate

that a BOR has occurred. The BOR circuit, if enabled,

will continue to operate while in Sleep or Idle modes

and will reset the device should VDD fall below the BOR

threshold voltage.

FIGURE 17-6:

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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