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TC820CPL 数据表(PDF) 18 Page - Microchip Technology |
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TC820CPL 数据表(HTML) 18 Page - Microchip Technology |
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18 / 28 page  TC820 DS21476B-page 18 © 2002 Microchip Technology Inc. FIGURE 5-4: SUGGESTED CRYSTAL OSCILLATOR CIRCUIT Where low cost is important, the RC circuit of Figure 5-5 can be used. The frequency of this circuit will be approximately: EQUATION 5-1: FIGURE 5-5: RC OSCILLATOR CIRCUIT Typical values are R = 10k Ω and C = 68pF. The resis- tor value should be ≥ 100kΩ. For accurate frequency measurement, an RC oscillator frequency of 40kHz is required. 5.5 System Timing All system timing is derived from the clock oscillator. The clock oscillator is divided by 2 prior to clocking the A/D counters. The clock is also divided by 8 to drive the buzzer, by 240 to generate the LCD backplane fre- quency, and by 40,000 for the frequency counter time- base. A simplified diagram of the system clock is shown in Figure 5-6. FIGURE 5-6: SYSTEM CLOCK GENERATION 5.6 Component Value Selection 5.6.1 AUTO-ZERO CAPACITOR - CAZ The value of the auto-zero capacitor (CAZ)has some influence on system noise. A 0.47 µF capacitor is rec- ommended; a low dielectric absorption capacitor (Mylar) is required. 5.6.2 REFERENCE VOLTAGE CAPACITOR - CREF The reference voltage capacitor used to ramp the inte- grator output voltage back to zero during the reference integrate cycle is stored on CREF.A 0.1µF capacitor is typical. A good quality, low leakage capacitor (such as Mylar) should be used. TC820 40kHz 38 39 5pF 10pF 22M Ω 470k Ω 37 TOSC = 0.3 RC 5pF 37 38 39 110k Ω 10pF 75pF TC820 A/D Counter TC820 ÷ 40,000 Counter Time-Base ÷ 240 LCD Backplane Driver ÷ 8 Buzzer ÷ 2 OSC1 OSC2 OSC3 XTAL Oscillator Components RC Oscillator Components |
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