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PIC16F010-E/MCSQTP 数据表(PDF) 61 Page - Microchip Technology |
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PIC16F010-E/MCSQTP 数据表(HTML) 61 Page - Microchip Technology |
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61 / 116 page  © 2007 Microchip Technology Inc. Preliminary DS41268C-page 59 PIC12F510/16F506 TABLE 10-2: CAPACITOR SELECTION FOR CRYSTAL OSCILLATOR – PIC12F510/16F506(2) 10.2.3 EXTERNAL CRYSTAL OSCILLATOR CIRCUIT Either a prepackaged oscillator or a simple oscillator circuit with TTL gates can be used as an external crystal oscillator circuit. Prepackaged oscillators provide a wide operating range and better stability. A well-designed crystal oscillator will provide good perfor- mance with TTL gates. Two types of crystal oscillator circuits can be used: one with parallel resonance or one with series resonance. Figure 10-3 shows implementation of a parallel reso- nant 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 potenti- ometers bias the 74AS04 in the linear region. This circuit could be used for external oscillator designs. FIGURE 10-3: EXTERNAL PARALLEL RESONANT CRYSTAL OSCILLATOR CIRCUIT Figure 10-4 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 10-4: EXTERNAL SERIES RESONANT CRYSTAL OSCILLATOR CIRCUIT 10.2.4 EXTERNAL RC OSCILLATOR For timing insensitive applications, the EXTRC device option offers additional cost savings. The EXTRC oscil- lator 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 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 components used. Figure 10-5 shows how the R/C combination is connected to the PIC12F510/16F506 devices. For REXT values below 5.0 k Ω, the oscillator operation may become unstable or stop completely. For very high REXT values (e.g., 1 M Ω), the oscillator becomes sensitive to noise, humidity and leakage. Thus, we recommend keeping REXT between 5.0 k Ω and 100 k Ω. Osc. Type Resonator Freq. Cap.Range C1 Cap. Range C2 LP 32 kHz(1) 15 pF 15 pF XT 200 kHz 1 MHz 4 MHz 47-68 pF 15 pF 15 pF 47-68 pF 15 pF 15 pF HS (3) 20 MHz 15-47 pF 15-47 pF Note 1: For VDD > 4.5V, C1 = C2 ≈ 30 pF is recommended. 2: These values are for design guidance only. Rs may be required to avoid over- driving crystals beyond the drive level specification. Since each crystal has its own characteristics, the user should con- sult the crystal manufacturer for appropri- ate values of external components. 3: PIC16F506 only. 20 pF +5V 20 pF 10k 4.7k 10k 74AS04 XTAL 10k 74AS04 CLKIN To Other Devices PIC12F510 PIC16F506 330 74AS04 74AS04 CLKIN To Other Devices XTAL 330 74AS04 0.1 mF PIC12F510 PIC16F506 |
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