Hardware layout and configuration UM1974
26/82 DocID028599 Rev 7
• HSE not used: PF0/PH1 and PF1/PH1 are used as GPIOs instead of as clock. The
configuration must be:
– SB148 and SB163 ON
– SB112 and SB149 (MCO) OFF
– SB8 and SB9 removed
6.8.2 OSC 32 KHz clock supply
There are three ways to configure the pins corresponding to low-speed clock (LSE):
• On-board oscillator (Default): X2 crystal. Refer to the Oscillator design guide for
STM8S, STM8A and STM32 microcontrollers Application note (AN2867) for oscillator
design guide for STM32 microcontrollers. It is recommended to use NX3214SA-
32.768KHZ-EXS00A-MU00525 (32.768KHz, 6pF load capacitance, 200ppm) from
Nihon Dempa Kogyo CO, LTD.
Note: For STM32F0 and STM32F3 Series it is recommended to use the low-drive-mode
configuration of the LSE (low-drive capability in LSEDRV register), due to the 6pF load
capacitance of the crystal on the board.
• Oscillator from external PC14: from external oscillator through the pin 25 of CN11
connector. The configuration must be:
– SB144 and SB145 ON
– R37 and R38 removed
• LSE not used: PC14 and PC15 are used as GPIOs instead of low-speed clock. The
configuration must be:
– SB144 and SB145 ON
– R37 and R38 removed
6.9 USART communication
The USART3 interface available on PD8 and PD9 of the STM32 can be connected either to
ST-LINK or to ST morpho connector. The choice is changed by setting the related solder
bridges. By default the USART3 communication between the target STM32 and the ST-
LINK is enabled, to support the virtual COM port for the mbed (SB5 and SB6 ON).
Table 9. USART3 pins
Pin
name
Function
Virtual COM port
(default configuration)
ST morpho connection
PD8 USART3 TX SB5 ON and SB7 OFF SB5 OFF and SB7 ON
PD9 USART3 RX SB6 ON and SB4 OFF SB6 OFF and SB4 ON
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