ST STM32L053 - USB Device Support; USART Configuration; OSC Clock Supply

UM1775 Rev 4 23/39
UM1775 Hardware layout
38
To minimize the noise, these pairs are dedicated to the linear touch sensor and touchkeys.
They are not connected to external headers. To design a touch sensing application, refer to
the following documentation and firmware:
• For details concerning the I/O ports, refer to the STM32L053C8T6 datasheet.
• For more detail concerning touch sensing application design and layout, refer to the
application note Design with surface sensors for touch sensing applications on MCUs
(AN4312).
• STM32 touch sensing library available from www.st.com/stm32l0-discovery.
The STM32L053C8T6 MCU controls the linear touch sensor and the touchkeys.
6.10 USB device support
The STM32L053C8T6 MCU is also used to drive the second USB Mini-B connector (USB
USER), which allows the use of the board as a USB Device. The STM32L053 Discovery
board can then act as a USB joystick, mouse, or other similar device. If both USBs are
connected, diodes D3 and D4 protect the board and use the power from USB ST-LINK or
USB USER (see Section 6.2.2: External power supply inputs: 5V_IN or USB USER CN3).
The board can be powered through this USB USER connector, in which case LED2 PWR
lights up, LED1 COM blinks and it can run an application in standalone mode.
The STM32L053C8T6 MCU controls the USB USER through PA11 and PA12.
6.11 USART configuration
The USART1 interface available on PA9 and PA10 of the STM32L053C8T6 can be
connected to the ST-LINK/V2-1 MCU to use the Virtual COM port function.
By default, the USART1 communication between the target STM32L053C8T6 and the
ST-LINK/V2-1 MCU is not enabled.
To use the Virtual COM port function with:
• The on-board STM32L053C8T6, set SB2 and SB3 ON. (SB14,15 must be OFF)
• An external MCU, remove solder from SB2 and SB3, solder a 2-pin header on JP3,
then connect RX and TX of the external MCU directly to RX and TX of JP3
6.12 OSC clock supply
If PH0 is only used as GPIO instead of as a clock, then SB21 is ON. (SB20 must be OFF)
MCO from ST-LINK/V2-1 (from MCO of the STM32L053C8T6)
This frequency cannot be changed, it is fixed at 8 MHz and connected to PH0-OSC_IN of
the STM32L053C8T6. The configuration needed is:
– SB20 ON and SB21 OFF
Oscillator from external PH0 (from external oscillator through pin 7 of the P3 connector)
The configuration needed is:
– SB21 ON and SB20 OFF
Note: Refer to application note AN2867 for STM32 microcontrollers oscillator design guide.