PIC16F57-ISLASHP datasheet(27/88 Pages) MICROCHIP

27 / 88 page

DS41213D-page 25

PIC16F5X

5.1

Power-on Reset (POR)

The PIC16F5X family of devices incorporate on-chip

Power-on Reset (POR) circuitry which provides an

internal chip Reset for most power-up situations. To

use this feature, the user merely ties the MCLR/VPP pin

to VDD. A simplified block diagram of the on-chip

Power-on Reset circuit is shown in Figure 5-1.

The Power-on Reset circuit and the Device Reset

Timer (Section 5.2) circuit are closely related. On

power-up, the Reset latch is set and the DRT is reset.

The DRT timer begins counting once it detects MCLR

to be high. After the time-out period, which is typically

18 ms, it will reset the Reset latch and thus end the on-

chip Reset signal.

A power-up example where MCLR is not tied to VDD is

shown in Figure 5-3. VDD is allowed to rise and stabilize

before bringing MCLR high. The chip will actually come

out of Reset TDRT msec after MCLR goes high.

In Figure 5-4, the on-chip Power-on Reset feature is

being used (MCLR and VDD are tied together). The VDD

is stable before the start-up timer times out and there is

no problem in getting a proper Reset. However,

Figure 5-5 depicts a problem situation where VDD rises

too slowly. The time between when the DRT senses a

high on the MCLR/VPP pin and the MCLR/VPP pin (and

VDD) actually reach their full value is too long. In this sit-

uation, when the start-up timer times out, VDD has not

reached the VDD (min) value and the chip is, therefore,

not ensured to function correctly. For such situations,

we recommend that external RC circuits be used to

achieve longer POR delay times (Figure 5-2).

For more information on the PIC16F5X POR, see

Application Note AN522, “Power-Up Considerations”

at www.microchip.com.

FIGURE 5-2:

EXTERNAL POWER-ON

RESET CIRCUIT (FOR

SLOW VDD POWER-UP)

Note 1: When the device starts normal operation

(exits

the

Reset

condition),

device

operating

parameters

(voltage,

fre-

quency, temperature, etc.) must be met to

ensure operation. If these conditions are

not met, the device must be held in Reset

until the operating conditions are met.

2: The POR is disabled when the device is

in Sleep.

MCLR

PIC16F5X

VDD

• External Power-on Reset circuit is required

only if VDD power-up is too slow. The diode D

helps discharge the capacitor quickly when

VDD powers down.

•R < 40 k

Ω is recommended to make sure that

voltage drop across R does not violate the

device electrical specification.

•R1 = 100

Ω to 1 kΩ will limit any current

flowing into MCLR from external capacitor C

in the event of MCLR pin breakdown due to

Electrostatic Discharge (ESD) or Electrical

Overstress (EOS).

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