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AN4488 Datasheet(PDF) 27 Page - STMicroelectronics |
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AN4488 Datasheet(HTML) 27 Page - STMicroelectronics |
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27 / 44 page  DocID026304 Rev 3 27/44 AN4488 Clocks 43 4.1.1 External source (HSE bypass) In this mode, an external clock source must be provided. It can have a frequency from 1 to 50 MHz (refer to STM32F4xxxx datasheets for actual max value). The external clock signal (square, sine or triangle) with a duty cycle of about 50%, has to drive the OSC_IN pin while the OSC_OUT pin must be left in the high impedance state (see Figure 19 and Figure 18). 4.1.2 External crystal/ceramic resonator (HSE crystal) The external oscillator frequency ranges from 4 to 26 MHz. The external oscillator has the advantage of producing a very accurate rate on the main clock. The associated hardware configuration is shown in Figure 19. Using a 25 MHz oscillator frequency is a good choice to get accurate Ethernet, USB OTG high-speed peripheral, and I2S. The resonator and the load capacitors have to be connected as close as possible to the oscillator pins in order to minimize output distortion and startup stabilization time. The load capacitance values must be adjusted according to the selected oscillator. For CL1 and CL2 it is recommended to use high-quality ceramic capacitors in the 5 pF-to- 25 pF range (typ.), designed for high-frequency applications and selected to meet the requirements of the crystal or resonator. CL1 and CL2, are usually the same value. The crystal manufacturer typically specifies a load capacitance that is the series combination of CL1 and CL2. The PCB and MCU pin capacitances must be included when sizing CL1 and CL2 (10 pF can be used as a rough estimate of the combined pin and board capacitance). Refer to the dedicated Application Note (AN2867 - Oscillator design guide for ST microcontrollers) and electrical characteristics sections in the datasheet of your product for more details. |
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