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PIC18F258-E/SOSQTP 数据表(PDF) 20 Page - Microchip Technology |
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PIC18F258-E/SOSQTP 数据表(HTML) 20 Page - Microchip Technology |
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20 / 402 page  PIC18FXX8 DS41159E-page 18 © 2006 Microchip Technology Inc. TABLE 2-2: CAPACITOR SELECTION FOR CRYSTAL OSCILLATOR 2.3 RC Oscillator For timing insensitive applications, the “RC” and “RCIO” device options offer 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 differ- ence 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 2-2 shows how the RC combination is connected. In the RC Oscillator mode, the oscillator frequency divided by 4 is available on the OSC2 pin. This signal may be used for test purposes or to synchronize other logic. FIGURE 2-2: RC OSCILLATOR MODE The RCIO Oscillator mode functions like the RC mode, except that the OSC2 pin becomes an additional general purpose I/O pin. The I/O pin becomes bit 6 of PORTA (RA6). Osc Type Crystal Freq Cap. Range C1 Cap. Range C2 LP 32.0 kHz 33 pF 33 pF 200 kHz 15 pF 15 pF XT 200 kHz 47-68 pF 47-68 pF 1.0 MHz 15 pF 15 pF 4.0 MHz 15 pF 15 pF HS 4.0 MHz 15 pF 15 pF 8.0 MHz 15-33 pF 15-33 pF 20.0 MHz 15-33 pF 15-33 pF 25.0 MHz 15-33 pF 15-33 pF These values are for design guidance only. See notes on this page. Crystals Used 32.0 kHz Epson C-001R32.768K-A ±20 PPM 200 kHz STD XTL 200.000KHz ±20 PPM 1.0 MHz ECS ECS-10-13-1 ±50 PPM 4.0 MHz ECS ECS-40-20-1 ±50 PPM 8.0 MHz EPSON CA-301 8.000M-C ±30 PPM 20.0 MHz EPSON CA-301 20.000M-C ±30 PPM Note 1: Recommended values of C1 and C2 are identical to the ranges tested (Table 2-1). 2: Higher capacitance increases the stability of the oscillator but also increases the start-up time. 3: Since each resonator/crystal has its own characteristics, the user should consult the resonator/crystal manufacturer for appropriate values of external components. 4: Rs may be required in HS mode, as well as XT mode, to avoid overdriving crystals with low drive level specification. Note: If the oscillator frequency divided by 4 signal is not required in the application, it is recommended to use RCIO mode to save current. OSC2/CLKO CEXT REXT PIC18FXX8 OSC1 FOSC/4 Internal Clock VDD VSS Recommended values: 3 k Ω ≤ REXT ≤ 100 kΩ CEXT > 20 pF |
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