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PIC16F526-I/P 数据表(PDF) 46 Page - Microchip Technology |
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PIC16F526-I/P 数据表(HTML) 46 Page - Microchip Technology |
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46 / 122 page  PIC16F526 DS41326D-page 46 2010 Microchip Technology Inc. TABLE 8-2: CAPACITOR SELECTION FOR CRYSTAL OSCILLATOR(2) 8.2.3 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 performance with TTL gates. Two types of crystal oscillator circuits can be used: one with parallel resonance, or one with series resonance. Figure 8-3 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 resistor provides the negative feedback for stability. The 10 k potentiometers bias the 74AS04 in the linear region. This circuit could be used for external oscillator designs. FIGURE 8-3: EXTERNAL PARALLEL RESONANT CRYSTAL OSCILLATOR CIRCUIT Figure 8-4 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 resistors provide the negative feedback to bias the inverters in their linear region. FIGURE 8-4: EXTERNAL SERIES RESONANT CRYSTAL OSCILLATOR CIRCUIT 8.2.4 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 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 components used. Figure 8-5 shows how the R/C combination is con- nected to the PIC16F526 device. For REXT values below 3.0 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 5.0 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 package lead frame capacitance. Section 14.0 “Electrical Characteristics” shows RC frequency variation from part-to-part due to normal process variation. The variation is larger for larger val- ues of R (since leakage current variation will affect RC frequency more for large R) and for smaller values of C (since variation of input capacitance will affect RC frequency more). 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 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 with low drive level specifi- cation. Since each crystal has its own characteristics, the user should consult the crystal manufacturer for appropriate values of external components. 20 pF +5V 20 pF 10k 4.7k 10k 74AS04 XTAL 10k 74AS04 CLKIN To Other Devices PIC16F526 330 74AS04 74AS04 PIC16F526 CLKIN To Other Devices XTAL 330 74AS04 0.1 mF |
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