PIC18F23K20-ESLASHML datasheet(55/456 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

DS41303G-page 55

PIC18F2XK20/4XK20

4.5

Device Reset Timers

PIC18F2XK20/4XK20

devices

incorporate

three

separate on-chip timers

that

help regulate

the

Power-on Reset process. Their main function is to

ensure that the device clock is stable before code is

executed. These timers are:

• Power-up Timer (PWRT)

• Oscillator Start-up Timer (OST)

• PLL Lock Time-out

4.5.1

POWER-UP TIMER (PWRT)

The

Power-up

Timer

(PWRT)

PIC18F2XK20/4XK20 devices is an 11-bit counter

which uses the LFINTOSC source as the clock input.

This

yields

approximate

time

interval

2048 x 32

s = 65.6 ms. While the PWRT is counting,

the device is held in Reset.

The power-up time delay depends on the LFINTOSC

clock and will vary from chip-to-chip due to temperature

and process variation. See DC parameter 33 for

details.

The PWRT is enabled by clearing the PWRTEN

Configuration bit.

4.5.2

OSCILLATOR START-UP TIMER

(OST)

The Oscillator Start-up Timer (OST) provides a 1024

oscillator cycle (from OSC1 input) delay after the

PWRT delay is over (parameter 33). This ensures that

the crystal oscillator or resonator has started and

stabilized.

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

HSPLL modes and only on Power-on Reset, or on exit

from all power-managed modes that stop the external

oscillator.

4.5.3

PLL LOCK TIME-OUT

With the PLL enabled in its PLL mode, the time-out

sequence following a Power-on Reset is slightly

different from other oscillator modes. A separate timer

is used to provide a fixed time-out that is sufficient for

the PLL to lock to the main oscillator frequency. This

PLL lock time-out (TPLL) is typically 2 ms and follows

the oscillator start-up time-out.

4.5.4

TIME-OUT SEQUENCE

On power-up, the time-out sequence is as follows:

After the POR pulse has cleared, PWRT time-out

is invoked (if enabled).

Then, the OST is activated.

The total time-out will vary based on oscillator

configuration and the status of the PWRT. Figure 4-3,

Figure 4-4, Figure 4-5, Figure 4-6 and Figure 4-7 all

depict time-out sequences on power-up, with the

Power-up Timer enabled and the device operating in

HS Oscillator mode. Figures 4-3 through 4-6 also

apply to devices operating in XT or LP modes. For

devices in RC mode and with the PWRT disabled, on

the other hand, there will be no time-out at all.

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

is kept low long enough, all time-outs will expire, after

which, bringing MCLR high will allow program

execution to begin immediately (Figure 4-5). This is

useful for testing purposes or to synchronize more than

one PIC18FXXK20 device operating in parallel.

TABLE 4-2:

TIME-OUT IN VARIOUS SITUATIONS

Oscillator

Configuration

Power-up(2) and Brown-out

Exit from

Power-Managed Mode

PWRTEN = 0

PWRTEN = 1

HSPLL

66 ms(1) + 1024 TOSC + 2 ms(2)

1024 TOSC + 2 ms(2)

HS, XT, LP

66 ms(1) + 1024 TOSC

1024 TOSC

EC, ECIO

66 ms(1)

——

RC, RCIO

66 ms(1)

——

INTIO1, INTIO2

66 ms(1)

——

Note 1: 66 ms (65.5 ms) is the nominal Power-up Timer (PWRT) delay.

2: 2 ms is the nominal time required for the PLL to lock.

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