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2002 Microchip Technology Inc.
Preliminary
DS30453D-page 17
PIC16C5X
4.4
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.
nected to the PIC16C5X. For REXT values below
2.2 k
, the oscillator operation may become unstable,
or stop completely. For very high REXT values
(e.g., 1 M
) the oscillator becomes sensitive to noise,
humidity and leakage. Thus, we recommend keeping
REXT between 3 k
and 100 k.
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.
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.
FIGURE 4-5:
RC OSCILLATOR MODE
Note:
If you change from this device to another
device, please verify oscillator characteris-
tics in your application.
VDD
REXT
CEXT
VSS
OSC1
Internal
clock
OSC2/CLKOUT
Fosc/4
PIC16C5X
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