Hardware layout and configuration UM2408
28/51 UM2408 Rev 2
Oscillator from external PH0: from an external oscillator through the pin 29 of the CN11
connector. The configuration must be:
–SB71 ON
–SB72 OFF
– SB3 and SB4 removed
• HSE not used: PH0 and PH1 are used as GPIOs instead of clocks. The configuration
must be:
– SB71 and SB90 ON
– SB72 (MCO) OFF
– SB3 and SB4 removed
6.9.2 OSC 32 KHz
There are three ways to configure the pins corresponding to the low-speed clock (LSE):
• Onboard oscillator (Default): X3 crystal. Refer to the application note Oscillator
design guide for STM8AF/AL/S and STM32 microcontrollers AN2867 for oscillator
design guide for STM32H7 microcontrollers. It is recommended to use NX3215SA-
32.768KHZ EXS00A-MU00525 (32.768 kHz, 6 pf load capacitance, 20 ppm) from
NDK.
• Oscillator from external PC14: from external oscillator through the pin 25 of CN11
connector. The configuration must be:
– SB66 and SB67 ON
– R41 and R42 removed
• LSE not used: PC14 and PC15 are used as GPIOs instead of low-speed clocks. The
configuration must be:
– SB66 and SB67 ON
– R41 and R42 removed
6.10 USART communication
The USART3 interface available on PD8 and PD9 of the STM32H7 can be connected either
to ST-LINK or to the ST morpho connector. The choice is changed by setting the related
solder bridges. By default, the USART3 communication between the target STM32H7 and
the STLINK is enabled, to support the Virtual COM port for the Mbed (SB16 and SB17 ON).
Table 12. USART3 pins
Pin
name
Function
Virtual COM port (Default
configuration)
ARDUINO
®
D0 and
D1
ST morpho
connection
PD8 USART3 TX
SB103 OFF, SB16 ON and
SB15 OFF
SB103 OFF, SB16
OFF and SB15 ON
SB103 ON, SB16
OFF, SB15 OFF
PD9 USART3 RX
SB104 OFF, SB17 ON and
SB94 OFF
SB104 OFF, SB17
OFF and SB94 ON
SB104 ON, SB17
OFF and SB94 OFF
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