PIC12F752 datasheet(139/204 Pages) MICROCHIP

139 / 204 page

 2011 Microchip Technology Inc.

Preliminary

DS41576A-page 139

PIC12F752/HV752

17.3.1

POWER-ON RESET (POR)

The on-chip POR circuit holds the chip in Reset until

VDD has reached a high enough level for proper

operation. To take advantage of the POR, simply

connect the MCLR pin through a resistor to VDD. This

will eliminate external RC components usually needed

to create Power-on Reset. A maximum rise time for

VDD

required.

See

Section 20.0

“Electrical

Specifications” for details. If the BOR is enabled, the

maximum rise time specification does not apply. The

BOR circuitry will keep the device in Reset until VDD

reaches VBOR (see Section 17.3.4 “Brown-out Reset

(BOR)”).

When the device starts normal operation (exits the

Reset condition), device operating parameters (i.e.,

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

ensure proper operation. If these conditions are not

met, the device must be held in Reset until the

operating conditions are met.

For additional information, refer to Application Note

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

17.3.2

MCLR

PIC12F752/HV752 has a noise filter in the MCLR

Reset path. The filter will detect and ignore small

pulses.

It should be noted that a WDT Reset does not drive

MCLR pin low.

Voltages applied to the MCLR pin that exceed its

specification can result in both MCLR Resets and

excessive current beyond the device specification

during the ESD event. For this reason, Microchip

recommends that the MCLR pin no longer be tied

directly to VDD. The use of an RC network, as shown in

Figure 17-2, is suggested.

An internal MCLR option is enabled by clearing the

MCLRE bit in the Configuration Word register. When

MCLRE = 0, the Reset signal to the chip is generated

internally. When the MCLRE = 1, the MCLR pin

becomes an external Reset input. In this mode, the

MCLR pin has a weak pull-up to VDD.

FIGURE 17-2:

RECOMMENDED MCLR

CIRCUIT

17.3.3

POWER-UP TIMER (PWRT)

The Power-up Timer provides a fixed 64 ms (nominal)

time-out on power-up only, from POR or Brown-out

Reset. The Power-up Timer operates from an internal

RC oscillator. For more information, see Section 4.2.2

“Internal Clock Mode”. The chip is kept in Reset as

long as PWRT is active. The PWRT delay allows the

VDD to rise to an acceptable level. A Configuration bit,

PWRTE, can disable (if set) or enable (if cleared or

programmed) the Power-up Timer. The Power-up

Timer should be enabled when Brown-out Reset is

enabled, although it is not required.

The Power-up Timer delay will vary from chip-to-chip

due to:

•VDD variation

• Temperature variation

• Process variation

See

parameters

for

details

(Section 20.0

“Electrical Specifications”).

Note:

The POR circuit does not produce an

internal Reset when VDD declines. To re-

enable the POR, VDD must reach Vss for

a minimum of 100

s.

Note:

Voltage spikes below VSS at the MCLR

pin, inducing currents greater than 80 mA,

may cause latch-up. Thus, a series resis-

tor of 50-100  should be used when

applying a “low” level to the MCLR pin,

rather than pulling this pin directly to VSS.

VDD

PIC®

MCLR

or greater)

0.1

F

(optional, not critical)

100



needed with capacitor)

SW1

(optional)

MCU

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