PIC16LC773-04SLASHSP datasheet(132/200 Pages) MICROCHIP

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PIC16C77X

DS30275A-page 132

Advance Information

12.4

Power-On Reset (POR)

A Power-on Reset pulse is generated on-chip when

VDD rise is detected (in the range of 1.5V - 2.1V). To

take advantage of the POR, just tie the MCLR pin

directly (or through a resistor) to VDD. This will elimi-

nate external RC components usually needed to create

a Power-on Reset. A maximum rise time for VDD is

specified. See Electrical Specifications for details. For

a slow rise time, see Figure 12-6.

Two delay timers have been provided which hold the

device in reset after a POR (dependant upon device

configuration) so that all operational parameters have

been met prior to releasing to device to resume/begin

normal operation.

When the device starts normal operation (exits the

reset condition), device operating parameters (voltage,

frequency, temperature,...) must be met to ensure oper-

ation. If these conditions are not met, the device must

be held in reset until the operating conditions are met.

Brown-out Reset may be used to meet the startup con-

ditions, or if necessary an external POR circuit may be

implemented to delay end of reset for as long as

needed.

FIGURE 12-6: EXTERNAL POWER-ON

RESET CIRCUIT (FOR SLOW

VDD POWER-UP)

12.5

Power-up Timer (PWRT)

The Power-up Timer provides a fixed 72 ms nominal

time-out on power-up type resets only. For a POR, the

PWRT is invoked when the POR pulse is generated.

For a BOR, the PWRT is invoked when the device exits

the reset condition (VDD rises above BOR trippoint).

The Power-up Timer operates on an internal RC oscil-

lator. The chip is kept in reset as long as the PWRT is

active. The PWRT’s time delay is designed to allow VDD

to rise to an acceptable level. A configuration bit is pro-

vided to enable/disable the PWRT for the POR only. For

a BOR the PWRT is always available regardless of the

configuration bit setting.

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

to VDD, temperature, and process variation. See DC

parameters for details.

12.6

Oscillator Start-up Timer (OST)

The Oscillator Start-up Timer (OST) provides 1024

oscillator cycle (from OSC1 input) delay after the

PWRT delay is over. This ensures that the crystal oscil-

lator or resonator has started and stabilized.

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

modes and only on a power-up type reset or a wake-up

from SLEEP.

12.7

Brown-Out Reset (BOR)

The Brown-out Reset module is used to generate a

reset when the supply voltage falls below a specified

trip voltage. The trip voltage is configurable to any one

of four voltages provided by the BORV1:BORV0 config-

uration word bits.

Configuration bit, BODEN, can disable (if clear/pro-

grammed) or enable (if set) the Brown-out Reset cir-

cuitry. If VDD falls below the specified trippoint for

greater than parameter #35 in the electrical specifica-

tions section, the brown-out situation will reset the chip.

A reset may not occur if VDD falls below the trippoint for

less than parameter #35. The chip will remain in Brown-

out Reset until VDD rises above BVDD. The Power-up

Timer will be invoked at that point and will keep the chip

in RESET an additional 72 ms. If VDD drops below

BVDD 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

BVDD, the Power-up Timer will again begin a 72 ms

time delay. Even though the PWRT is always enabled

when brown-out is enabled, the PWRT configuration

word bit should be cleared (enabled) when brown-out is

enabled.

Note 1: External Power-on Reset circuit is required

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

diode D helps discharge the capacitor

quickly when VDD powers down.

2: R < 40 k

Ω is recommended to make sure

that voltage drop across R does not violate

the device’s electrical specification.

3: R1 = 100

Ω to 1 kΩ will limit any current

flowing into MCLR from external capacitor

C in the event of MCLR/VPP pin break-

down due to Electrostatic Discharge

(ESD) or Electrical Overstress (EOS).

VDD

MCLR

PIC16C77X

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