Microchip Technology dsPIC30F - Crystal Oscillators;Ceramic Resonators

dsPIC30F Family Reference Manual
DS70054C-page 7-10 © 2004 Microchip Technology Inc.
7.6 Crystal Oscillators/Ceramic Resonators
In XT, XTL and HS modes, a crystal or ceramic resonator is connected to the OSC1 and OSC2
pins to establish oscillation (Figure 7-3). The dsPIC30F oscillator design requires the use of a
parallel cut crystal. Using a series cut crystal may give a frequency out of the crystal
manufacturer’s specifications.
Figure 7-3: Crystal or Ceramic Resonator Operation (XT, XT or HS Oscillator Mode)
7.6.1 Oscillator/Resonator Start-up
As the device voltage increases from VSS, the oscillator will start its oscillations.The time required
for the oscillator to start oscillating depends on many factors. These include:
• Crystal/resonator frequency
• Capacitor values used (C1 and C2 in Figure 7-3)
• Device V
DD rise time
• System temperature
• Series resistor value and type if used (Rs in Figure 7-3)
• Oscillator mode selection of device (selects the gain of the internal oscillator inverter)
• Crystal quality
• Oscillator circuit layout
• System noise
Figure 7-4 shows a plot of a typical oscillator/resonator start-up.
Figure 7-4: Example Oscillator/Resonator Start-up Characteristics
C1
(3)
C2
(3)
XTAL
OSC2
R
S
(1)
OSC1
RF
(2)
Sleep
To Internal Logic
dsPIC30FXXXX
Note 1: A series resistor, Rs, may be required for AT strip cut crystals.
2: The internal feedback resistor, R
F, is typically in the range of 2 to
10 MΩ.
3: See Section 7.7 “Determining Best Values for Crystals, Clock
Mode, C1, C2 and Rs”.
Voltage
Crystal Start-up Time
Time
Device V
DD
Maximum VDD of System
0V
VIL
VIH