Hardware and layout UM1525
18/41 Doc ID 022910 Rev 2
4.7 OSC clock
4.7.1 OSC clock supply
PF0 and PF1 can be used as GPIO or as HSE oscillator. By default these I/Os are
configured as GPIO, so SB16 and SB17 are closed, SB18 is open and R22, R23, C13 and
C14 are not populated.
An external HSE clock can be provided to the MCU in three ways:
● MCO from ST-LINK. From MCO of the STM32F103. This frequency cannot be
changed, it is fixed at 8
MHz and connected to PF0-OSC_IN of the STM32F051R8T6.
Configuration needed:
– SB16, SB18 CLOSED
–R22, R23 removed
– SB17 OPEN
● Oscillator onboard. From X2 crystal (not provided). For typical frequencies and its
capacitors and resistors, please refer to the STM32F051R8T6 Datasheet.
Configuration needed:
– SB16, SB17 SB18 OPEN
– R22, R23, C13, C14 soldered
● Oscillator from external PF0. From external oscillator through pin 7 of the P1
connector. Configuration needed:
– SB16, SB17 CLOSED
– SB18 OPEN
– R22 and R23 removed
4.7.2 OSC 32 KHz clock supply
PC14 and PC15 can be used as GPIO or as LSE oscillator. By default these I/Os are
configured as GPIO, so SB20 & SB21 are closed and X3, R24, R25 are not populated.
An external LSE clock can be provided to the MCU in two ways:
● Oscillator onboard. From X3 crystal (not provided). Configuration needed:
– SB20, SB21 OPEN
– C15, C16, R24 and R25 soldered.
● Oscillator from external PC14. From external oscillator trough the pin 5 of P1
connector. Configuration needed:
– SB20, SB21 CLOSED
– R24 and R25 removed
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