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PIC12LC671 数据表(PDF) 55 Page - Microchip Technology |
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PIC12LC671 数据表(HTML) 55 Page - Microchip Technology |
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55 / 129 page  © 1999 Microchip Technology Inc. DS30561B-page 55 PIC12C67X 9.2.3 EXTERNAL CRYSTAL OSCILLATOR CIRCUIT Either a pre-packaged oscillator or a simple oscillator circuit with TTL gates can be used as an external crystal oscillator circuit. Pre-packaged oscillators provide a wide operating range and better stability. A well-designed crystal oscillator will provide good performance with TTL gates. Two types of crystal oscillator circuits can be used; one with parallel resonance or one with series resonance. Figure 9-3 shows implementation of a parallel resonant oscillator circuit. The circuit is designed to use the fundamental frequency of the crystal. The 74AS04 inverter performs the 180-degree phase shift that a parallel oscillator requires. The 4.7 k Ω resistor provides the negative feedback for stability. The 10 k Ω potentiometers bias the 74AS04 in the linear region. This circuit could be used for external oscillator designs. FIGURE 9-3: EXTERNAL PARALLEL RESONANT CRYSTAL OSCILLATOR CIRCUIT Figure 9-4 shows a series resonant oscillator circuit. This circuit is also designed to use the fundamental frequency of the crystal. The inverter performs a 180- degree phase shift in a series resonant oscillator circuit. The 330 Ω resistors provide the negative feedback to bias the inverters in their linear region. FIGURE 9-4: EXTERNAL SERIES RESONANT CRYSTAL OSCILLATOR CIRCUIT 20 pF +5V 20 pF 10k 4.7k 10k 74AS04 XTAL 10k 74AS04 PIC12C67X CLKIN To Other Devices 330 74AS04 74AS04 PIC12C67X CLKIN To Other Devices XTAL 330 74AS04 0.1 µF 9.2.4 EXTERNAL 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 resistor (REXT) and capacitor (CEXT) values, and the 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 CEXT values. The user also needs to take into account variation due to tolerance of external R and C components used. Figure 9-5 shows how the R/C combination is connected to the PIC12C67X. For REXT values below 2.2 k Ω, the oscillator operation may become unstable or stop completely. For very high REXT values (i.e., 1 M Ω), the oscillator becomes sensitive to noise, humidity and leakage. Thus, we recommend keeping REXT between 3 k Ω and 100 kΩ. 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 package lead frame capacitance. 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). FIGURE 9-5: EXTERNAL RC OSCILLATOR MODE VDD REXT CEXT VSS OSC1 Internal clock PIC12C67X N OSC2/CLKOUT FOSC/4 |
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