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PIC16C925T-S/CL 数据表(PDF) 102 Page - Microchip Technology |
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PIC16C925T-S/CL 数据表(HTML) 102 Page - Microchip Technology |
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102 / 182 page  PIC16C925/926 DS39544A-page 100 Preliminary 2001 Microchip Technology Inc. 12.2.3 EXTERNAL CRYSTAL OSCILLATOR CIRCUIT Either a prepackaged oscillator can be used, or a sim- ple oscillator circuit with TTL gates can be built. 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 series resonance, or one with parallel resonance. Figure 12-3 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 par- allel oscillator requires. The 4.7 k Ω resistor provides the negative feedback for stability. The 10 k Ω potenti- ometer biases the 74AS04 in the linear region. This could be used for external oscillator designs. FIGURE 12-3: EXTERNAL PARALLEL RESONANT CRYSTAL OSCILLATOR CIRCUIT Figure 12-4 shows a series resonant oscillator circuit. This circuit is also designed to use the fundamental fre- quency of the crystal. The inverter performs a 180-degree phase shift in a series resonant oscillator circuit. The 330 k Ω resistors provide the negative feed- back to bias the inverters in their linear region. FIGURE 12-4: EXTERNAL SERIES RESONANT CRYSTAL OSCILLATOR CIRCUIT 12.2.4 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 compo- nents used. Figure 12-5 shows how the R/C combina- tion is connected to the PIC16CXXX. For REXT values below 2.2 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 to keep 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 pack- age lead frame capacitance. See characterization data for desired device for RC fre- quency variation from part to part, due to normal pro- cess variation. 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). See characterization data for desired device for varia- tion of oscillator frequency, due to VDD for given REXT/CEXT values, as well as frequency variation due to operating temperature for given R, C, and VDD values. The oscillator frequency, divided by 4, is available on the OSC2/CLKOUT pin, and can be used for test pur- poses or to synchronize other logic (see Figure 1-2 for waveform). FIGURE 12-5: RC OSCILLATOR MODE 20 pF +5V 20 pF 10k 4.7k 10k 74AS04 XTAL 10k 74AS04 CLKIN To Other Devices PIC16CXXX 330 k Ω 74AS04 74AS04 PIC16CXXX CLKIN To Other Devices XTAL 330 k Ω 74AS04 0.1 µF OSC2/CLKOUT CEXT VDD REXT VSS PIC16CXXX OSC1 FOSC/4 Internal Clock |
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