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AN2867 Datasheet(PDF) 12 Page - STMicroelectronics |
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AN2867 Datasheet(HTML) 12 Page - STMicroelectronics |
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12 / 41 page  Pierce oscillator design AN2867 12/41 DocID15287 Rev 9 3.2 RF feedback resistor In most STMicroelectronics microcontrollers, RF is embedded in the oscillator circuitry. Its role is to make the inverter act as an amplifier. The feedback resistor is connected between Vin and Vout so as to bias the amplifier at Vout = Vin and force it to operate in the linear region (shaded area in Figure 6). The amplifier amplifies the noise (for example, the thermal noise of the crystal) within the range of serial to parallel frequency (Fa, Fa). This noise causes the oscillations to start up. In some cases, if RF is removed after the oscillations have stabilized, the oscillator continues to operate normally. Figure 6. Inverter transfer function Table 3 provides typical values of RF. Table 3. Typical feedback resistor values for given frequencies Frequency Feedback resistor range 32.768 kHz 10 to 25 M Ω 1 MHz 5 to 10 M Ω 10 MHz 1 to 5 M Ω 20 MHz 470 k Ω to 5 MΩ |
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