Hardware layout and configuration UM2435
32/48 UM2435 Rev 2
If this sequence is not respected, the board may be powered by VBUS first from STLINK,
with some risks:
• If more than 500 mA are needed by the board, the PC may be damaged, or the current
can be limited by the PC: as a consequence the board will be not correctly powered.
• 500 mA is requested by enumeration (since SB37 must be OFF), this request can be
rejected and enumeration won’t succeed, consequently the board will be not powered
(LED5 remains OFF).
In some cases it can be interesting to use the 3V3 (CN6 pin 4 or CN7 pin 16) directly as
power input, for instance when the 3.3
V is provided by an extension board. When Nucleo is
powered by 3V3, the ST-LINK is not powered, thus programming and debug features are
unavailable.
7.7 OSC clock sources
• LSE: 32.768 kHz external oscillator, for accurate RTC and calibration with other
embedded RC oscillators
• HSE: high quality 32 MHz external oscillator with trimming, needed by the RF
subsystem
7.7.1 LSE: OSC 32 kHz clock supply
There are three ways to configure the pins corresponding to low-speed clock (LSE):
1. LSE on-board oscillator X2 crystal (default configuration) 32.768 kHz, 7 pF, 20 ppm.
Refer to application note AN2867 “Oscillator design guide for STM8AF/AL/S and
STM32 microcontrollers”, available on www.st.com. It is recommended to use
NX2012SA manufactured by NDK.
2. Oscillator from external to PC14 input: from external oscillator through pin 25 of CN7
connector. The following configuration is needed:
– SB45 and SB46 ON
– X2, C6 and C7 removed
3. LSE not used: PC14 and PC15 are used as GPIOs instead of low speed clock. The
following configuration is needed:
– SB45 and SB46 ON
– X2, C6 and C7 removed
7.7.2 OSC clock supply
The HSE on board oscillator 32 MHz X1 crystal is provided for RF activities with tuning
capacitors. Refer to STM32 microcontroller datasheets, and to AN2867 for oscillator design.
It is recommended to use NX2016SA 32 MHz EXS00A-CS06654 manufactured by NDK.
SB44 and SB43 must be open.
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