Datasheet

PIC17C7XX
DS30289C-page 20  1998-2013 Microchip Technology Inc.
4.1.6 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
(R
EXT) 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 C
EXT 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 R
EXT values below
2.2 k, the oscillator operation may become unstable,
or stop completely. For very high R
EXT values (e.g.
1M), the oscillator becomes sensitive to noise,
humidity and leakage. Thus, we recommend to keep
R
EXT between 3 k and 100 k.
Although the oscillator will operate with no external
capacitor (C
EXT = 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 V
DD 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
4.1.6.1 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
#D032 and parameter #D042 in the electrical specifica-
tion section). The time required for the RC to start oscil-
lating depends on many factors. These include:
• Resistor value used
• Capacitor value used
• Device V
DD rise time
• System temperature
VDD
REXT
CEXT
VSS
OSC1
Internal
Clock
OSC2/CLKOUT
F
OSC/4
PIC17CXXX