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PC87413 Datasheet(PDF) 143 Page - National Semiconductor (TI) |
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PC87413 Datasheet(HTML) 143 Page - National Semiconductor (TI) |
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143 / 257 page  8.0 Real-Time Clock (RTC) (Continued) Revision 1.2 143 www.national.com Table 35. Crystal Oscillator Circuit Components External Elements Choose C1 and C2 capacitors (see Figure 39) to match the crystal’s load capacitance. The load capacitance CL “seen” by crystal Y is comprised of C1 in series with C2 and in parallel with the parasitic capacitance of the circuit. The parasitic ca- pacitance is caused by the chip package, board layout and socket (if any) and can vary from 0 to 8 pF. The rule of thumb in choosing these capacitors is: CL = (C1 * C2) / (C1 + C2) + CPARASITIC Oscillator Start-Up The oscillator starts to generate 32.768 KHz pulses to the RTC after about t32KW from when VBAT is higher than VBATMIN (2.4V) or VSB is higher than VSBON (2.5V). The oscillation amplitude on the X2C pin stabilizes to its final value (approximately 0.4V peak-to-peak around 0.7V DC) in about 1 sec. C1 can be trimmed to achieve precisely 32.768 KHz. For highly accurate timekeeping, use crystal and capacitors with low tolerance and temperature coefficients. 8.2.4 External Oscillator 32.768 KHz can be applied from an external clock source, as shown in Figure 40. Figure 40. External Oscillator Connections Connections Connect the clock to the 32KCLKIN pin, leaving the oscillator output, 32KX2, unconnected. Component Parameters Values Tolerance Crystal Resonance Frequency 32.768 KHz Parallel mode User defined Type N-Cut or XY-bar Serial Resistance 40 K Ω Max Quality Factor, Q 35000 Min Shunt Capacitance 2 pF Max Load Capacitance, CL 9-13 pF Temperature Coefficient User-defined Resistor R1 Resistance 20 M Ω 5% Resistor R2 Resistance 510 K Ω 5% Capacitor C1 Capacitance 10 pF 5% Capacitor C2 Capacitance 10 pF 5% 32.768 KHz Clock Generator Internal External 32KCLKIN / 32KX2 NC To other modules Battery VBAT B1 CF CF OUT PWR CF = 0.1 µF 32KX1 |
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