PIC16F72SLASHML datasheet(64/136 Pages) MICROCHIP

64 / 136 page

PIC16F72

DS39597B-page 62

 2002 Microchip Technology Inc.

TABLE 11-2:

CAPACITOR SELECTION FOR

CRYSTAL OSCILLATOR (FOR

DESIGN GUIDANCE ONLY)

11.2.3

RC OSCILLATOR

For timing insensitive applications, the “RC” device

option offers additional cost savings. The RC oscillator

frequency is a function of the supply voltage, the resis-

tor (REXT) and capacitor (CEXT) values, and the operat-

ing temperature. In addition to this, the oscillator

frequency will vary from unit to unit due to normal pro-

cess parameter variation. Furthermore, the difference

in lead frame capacitance between package types will

also affect the oscillation frequency, especially for low

CEXT values. The user also needs to take into account

variation due to tolerance of external R and C com-

ponents used. Figure 11-3 shows how the R/C

combination is connected to the PIC16F72.

FIGURE 11-3:

RC OSCILLATOR MODE

11.3

RESET

The PIC16F72 differentiates between various kinds of

RESET:

• Power-on Reset (POR)

• MCLR Reset during normal operation

• MCLR Reset during SLEEP

• WDT Reset (during normal operation)

• WDT Wake-up (during SLEEP)

• Brown-out Reset (BOR)

Some registers are not affected in any RESET condi-

tion. Their status is unknown on POR and unchanged

in any other RESET. Most other registers are reset to a

“RESET state” on Power-on Reset (POR), on the

MCLR and WDT Reset, on MCLR Reset during

SLEEP, and Brown-out Reset (BOR). They are not

affected by a WDT Wake-up, which is viewed as the

resumption of normal operation. The TO and PD bits

are set or cleared differently in different RESET situa-

tions, as indicated in Table 11-4. These bits are used in

software to determine the nature of the RESET. See

Table 11-6 for a full description of RESET states of all

registers.

A simplified block diagram of the on-chip RESET circuit

is shown in Figure 11-4.

Osc Type

Crystal

Freq

Typical Capacitor Values

Tested:

32 kHz

33 pF

200 kHz

15 pF

200 kHz

56 pF

1 MHz

15 pF

4 MHz

15 pF

4 MHz

15 pF

8 MHz

15 pF

20 MHz

15 pF

Capacitor values are for design guidance only.

These capacitors were tested with the crystals listed

below for basic start-up and operation. These values

were not optimized.

Different capacitor values may be required to produce

acceptable oscillator operation. The user should test

the performance of the oscillator over the expected

VDD and temperature range for the application.

See the notes following this table for additional

information.

Note 1: Higher capacitance increases the stability

of oscillator, but also increases the

start-up time.

2: Since each resonator/crystal has its own

characteristics, the user should consult the

resonator/crystal manufacturer for appro-

priate values of external components.

3: Rs may be required in HS mode, as well

as XT mode, to avoid overdriving crystals

with low drive level specification.

4: Always verify oscillator performance over

the VDD and temperature range that is

expected for the application.

OSC2/CLKO

CEXT

REXT

PIC16F72

OSC1

FOSC/4

Internal

Clock

VDD

VSS

Recommended values:

3 k

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20 pF

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