PIC17C75X
4.1.5
RC OSCILLATOR
4.1.5.1
RC START-UP
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 kΩ, the oscillator operation may become unstable,
or stop completely. For very high Rext values (e.g.
1 MΩ), the oscillator becomes sensitive to noise,
humidity and leakage. Thus, we recommend to keep
Rext between 3 kΩ and 100 kΩ.
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
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
PIC17CXXX
Rext
Internal
clock
OSC1
Cext
VSS
OSC2/CLKOUT
Fosc/4
DS30264A-page 18
Preliminary
1997 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
