PIC16F627A-I/SO【PDF 6/180 页】IC MCU FLASH 1KX14 EEPROM 18SOIC

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2009 Microchip Technology Inc.

DS40044G-page 103

PIC16F627A/628A/648A

14.4

Power-on Reset (POR), Power-up

Timer (PWRT), Oscillator Start-up

Timer (OST) and Brown-out Reset

(BOR)

14.4.1

POWER-ON RESET (POR)

The on-chip POR holds the part in Reset until a VDD

rise is detected (in the range of 1.2-1.7V). A maxi-

mum rise time for VDD is required. See Section 17.0

“Electrical Specifications”

for details.

The POR circuit does not produce an internal Reset

when VDD declines.

When the device starts normal operation (exits the

Reset

condition),

device

operating

parameters

(voltage, frequency, temperature, etc.) must be met to

ensure proper operation. If these conditions are not

met, the device must be held in Reset via MCLR, BOR

or PWRT until the operating conditions are met.

For additional information, refer to Application Note

AN607 “Power-up Trouble Shooting” (DS00607).

14.4.2

POWER-UP TIMER (PWRT)

The PWRT provides a fixed 72 ms (nominal) time out

on power-up (POR) or if enabled from a Brown-out

Reset. The PWRT operates on an internal RC oscilla-

tor. The chip is kept in Reset as long as PWRT is active.

The PWRT delay allows the VDD to rise to an accept-

able level. A configuration bit, PWRTE can disable (if

set) or enable (if cleared or programmed) the PWRT. It

is recommended that the PWRT be enabled when

Brown-out Reset is enabled.

The power-up time delay will vary from chip-to-chip and

due to VDD, temperature and process variation. See

DC parameters Table 17-7 for details.

14.4.3

OSCILLATOR START-UP TIMER

(OST)

The OST provides a 1024 oscillator cycle (from OSC1

input) delay after the PWRT delay is over. Program

execution will not start until the OST time out is

complete. This ensures that the crystal oscillator or

resonator has started and stabilized.

The OST time out is invoked only for XT, LP and HS

modes and only on Power-on Reset or wake-up from

Sleep. See Table 17-7.

14.4.4

BROWN-OUT RESET (BOR)

The PIC16F627A/628A/648A have on-chip BOR

circuitry. A configuration bit, BOREN, can disable (if

clear/programmed) or enable (if set) the BOR circuitry.

If VDD falls below VBOR for longer than TBOR, the

brown-out situation will reset the chip. A Reset is not

assured if VDD falls below VBOR for shorter than TBOR.

VBOR and TBOR are defined in Table 17-2 and

Table 17-7, respectively.

On any Reset (Power-on, Brown-out, Watchdog, etc.),

the chip will remain in Reset until VDD rises above

VBOR (see Figure 14-7). The Power-up Timer will now

be invoked, if enabled, and will keep the chip in Reset

an additional 72 ms.

If VDD drops below VBOR while the Power-up Timer is

running, the chip will go back into a Brown-out Reset

and the Power-up Timer will be re-initialized. Once VDD

rises above VBOR, the Power-Up Timer will execute a

72 ms Reset. Figure 14-7 shows typical brown-out

situations.

FIGURE 14-7:

BROWN-OUT SITUATIONS WITH PWRT ENABLED

72 ms

VBOR

VDD

Internal

Reset

VBOR

VDD

Internal

Reset

72 ms

<72 ms

72 ms

VBOR

VDD

Internal

Reset

≥ TBOR

Note:

72 ms delay only if PWRTE bit is programmed to ‘0’.

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