PIC17C752 datasheet(18/320 Pages) MICROCHIP

18 / 320 page

PIC17C75X

DS30264A-page 18

Preliminary

4.1.5

RC OSCILLATOR

For timing insensitive applications, the RC device

option offers additional cost savings. RC oscillator fre-

quency is a function of the supply voltage, the resistor

(Rext) and capacitor (Cext) values, and the operating

temperature. In addition to this, oscillator frequency will

vary from unit to unit due to normal process parameter

variation. Furthermore, the difference in lead frame

capacitance between package types will also affect

oscillation frequency, especially for low Cext values.

The user also needs to take into account variation due

to tolerance of external R and C components used.

Figure 4-7 shows how the R/C combination is con-

nected to the PIC17CXXX. For Rext values below

2.2 k

Ω, the oscillator operation may become unstable,

or stop completely. For very high Rext values (e.g.

1 M

Ω), the oscillator becomes sensitive to noise,

humidity and leakage. Thus, we recommend to keep

Rext between 3 k

Ω and 100 kΩ.

Although the oscillator will operate with no external

capacitor (Cext = 0 pF), we recommend using values

above 20 pF for noise and stability reasons. With little

or no external capacitance, oscillation frequency can

vary dramatically due to changes in external capaci-

tances, such as PCB trace capacitance or package

lead frame capacitance.

See Section 21.0 for RC frequency variation from part

to part due to normal process variation. The variation

is larger for larger R (since leakage current variation

will affect RC frequency more for large R) and for

smaller C (since variation of input capacitance will

affect RC frequency more).

See Section 21.0 for variation of oscillator frequency

due to VDD for given Rext/Cext values as well as fre-

quency variation due to operating temperature for

given R, C, and VDD values.

The oscillator frequency, divided by 4, is available on

the OSC2/CLKOUT pin, and can be used for test pur-

poses or to synchronize other logic (see Figure 4-8 for

waveform).

FIGURE 4-7: RC OSCILLATOR MODE

VDD

Rext

Cext

VSS

OSC1

Internal

clock

OSC2/CLKOUT

Fosc/4

PIC17CXXX

4.1.5.1

RC START-UP

As the device voltage increases, the RC will immedi-

ately start its oscillations once the pin voltage levels

meet the input threshold speciﬁcations (parameter

number D032 and D042 in the electrical speciﬁcation

section). The time required for the RC to start oscillat-

ing depends on many factors. These include:

• Resistor value used

• Capacitor value used

• Device VDD rise time

• System temperature

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