dsPIC30F Family Reference Manual
DS70054C-page 7-14 © 2004 Microchip Technology Inc.
7.9 External RC Oscillator
For timing insensitive applications, the ERC and ERCIO modes of the Primary oscillator offer
additional cost savings. The RC oscillator frequency is a function of the:
• Supply voltage
• External resistor (R
EXT) values
• External capacitor (C
EXT) values
• Operating temperature
In addition to this, the 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 the oscillation frequency, especially for low C
EXT values. The user also
needs to take into account variation due to tolerance of external R
EXT and CEXT components
used. Figure 7-7 shows how the RC combination is connected. For R
EXT values below 2.2 kΩ,
oscillator operation may become unstable or stop completely. For very high R
EXT values (e.g.,
1MΩ), the oscillator becomes sensitive to noise, humidity and leakage. Thus, it is recommended
that a R
EXT value between 3 kΩ and 100 kΩ is used.
Figure 7-7: ERC Oscillator Mode
Although the oscillator will operate with no external capacitor (C
EXT = 0 pF), a value above 20 pF
should be used for noise and stability reasons. With no or a small external capacitance, the
oscillation frequency can vary dramatically due to changes in external capacitances, such as
PCB trace capacitance and package lead frame capacitance.
The oscillator frequency, divided by 4, is available on the OSC2/CLKO pin, and can be used for
test purposes or to synchronize other logic.
Note: An external clock source should not be connected to the OSC1 pin when the
oscillator is configured for ERC or ERCIO modes.
OSC2
CEXT
VDD
REXT
VSS
dsPIC30F
OSC1
F
OSC/4
Internal
Clock
To Next Page
Loading...
