PIC12C67X datasheet(58/129 Pages) MICROCHIP

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PIC12C67X

DS30561B-page 58

9.4

Power-on Reset (POR), Power-up

Timer (PWRT) and Oscillator Start-up

Timer (OST)

9.4.1

POWER-ON RESET (POR)

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

VDD has reached a high enough level for proper opera-

tion. To take advantage of the POR, just tie the MCLR

pin through a resistor to VDD. This will eliminate exter-

nal RC components usually needed to create a Power-

on Reset. A maximum rise time for VDD is specified.

See Electrical Specifications for details.

When the device starts normal operation (exits the

reset condition), device operating parameters (voltage,

frequency, temperature, ...) 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.

For additional information, refer to Application Note

AN607, "

Power-up Trouble Shooting."

9.4.2

POWER-UP TIMER (PWRT)

The Power-up Timer provides a fixed 72 ms nominal

time-out on power-up only, from the POR. The Power-

up Timer operates on an internal RC oscillator. The

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

PWRT’s time delay allows VDD to rise to an acceptable

level. A configuration bit is provided to enable/disable

the PWRT.

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

to VDD, temperature and process variation. See

Table 11-4.

9.4.3

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 Power-on Reset or wake-up from

SLEEP.

9.4.4

TIME-OUT SEQUENCE

On power-up, the Time-out Sequence is as follows:

first, PWRT time-out is invoked after the POR time

delay has expired; then, OST is activated. The total

time-out will vary, based on oscillator configuration and

the status of the PWRT. For example, in RC mode with

the PWRT disabled, there will be no time-out at all.

Figure 9-7, Figure 9-8, and Figure 9-9 depict time-out

sequences on power-up.

Since the time-outs occur from the POR pulse, if MCLR

is kept low long enough, the time-outs will expire. Then

bringing MCLR high will begin execution immediately

(Figure 9-9). This is useful for testing purposes or to

synchronize more than one PIC12C67X device operat-

ing in parallel.

9.4.5

POWER CONTROL (PCON)/STATUS

The Power Control/Status Register, PCON (address

8Eh), has one bit. See Register 4-6 for register.

Bit1 is POR (Power-on Reset). It is cleared on a Power-

on Reset and is unaffected otherwise. The user sets

this bit following a Power-on Reset. On subsequent

resets, if POR is ‘0’, it will indicate that a Power-on

Reset must have occurred.

TABLE 9-4:

TIME-OUT IN VARIOUS SITUATIONS

TABLE 9-5:

STATUS/PCON BITS AND THEIR SIGNIFICANCE

Legend: u = unchanged, x = unknown.

Oscillator Configuration

Power-up

Wake-up from SLEEP

PWRTE = 0

PWRTE = 1

XT, HS, LP

72 ms + 1024TOSC

1024TOSC

INTRC, EXTRC

72 ms

—

POR

Power-on Reset

Illegal, TO is set on POR

Illegal, PD is set on POR

WDT Reset

WDT Wake-up

MCLR Reset during normal operation

MCLR Reset during SLEEP or interrupt wake-up from SLEEP

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