RFPIC12C509AF datasheet(47/104 Pages) MICROCHIP

47 / 104 page

Preliminary

DS70031A-page 45

rfPIC12C509AG/509AF

8.2.3

EXTERNAL 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 compo-

nents used.

Figure 8-4 shows how the R/C combination is con-

nected to the rfPIC12C509AG/509AF. 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 keeping

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 no or

small external capacitance, the oscillation frequency

can vary dramatically due to changes in external

capacitances, such as PCB trace capacitance or pack-

age lead frame capacitance.

The Electrical Specifications sections show RC fre-

quency variation from part to part due to normal pro-

cess 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).

Also, see the Electrical Specifications sections for vari-

ation of oscillator frequency due to VDD for given REXT/

CEXT values as well as frequency variation due to oper-

ating temperature for given R, C, and VDD values.

FIGURE 8-4:

EXTERNAL RC

OSCILLATOR MODE

VDD

REXT

CEXT

VSS

OSC1

Internal

clock

rfPIC12C509AG/509AF

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