STM32U0 Notes

• Datasheets
  • DS14581 STM32U031x4/6/8
  • DS14548 STM32U073x8/B/C
  • DS14463 STM32U083xC
• Application Notes
  • AN5938 Getting started with STM32U0 MCUs hardware development
  • AN2606 STM32 microcontroller system memory boot mode
• Reference Manual
  • RM0503 Reference manual

1. Nucleo-U083RC (STM32U083RCT6)
   • Purchasing
     • S$21.68 at element14
   • User Manuals
     • UM1727 Getting started with STM32 Nucleo board software development tools
     • UM3256 STM32 Nucleo-64 boards (MB1932)
2. STM32U083C-DK (STM32U083MC)
   • Purchasing
     • S$37.06 at element14
   • User Manuals
     • UM3292 Discovery kit with STM32U083MC MCU
     • UM3313 STM32CubeU0 STM32U083C-DK demonstration firmware

• VDDx
  • 1.71 ≤ VDDx ≤ 3.60 V (absolute maximum: 4.00 V)
  • Nucleo-083RC LEDs become very dim when 3.3V net falls below 1.80 V
• Brownout Reset¹⁾
  • Selectable: 1.7, 2.0, 2.2, 2.5 or 2.8 V

In order to use the native USB for programming on pins PA11 and PA12, the following settings must be done in STM32CubeProgrammer → Option bytes → User Configuration.

Option byte name	Value	Description	Notes
NBOOT_SEL	0 (Unchecked)	BOOT0 pin (legacy mode)	Factory setting is 1. You must change it to 0 to keep USB DFU programming capability.
nBOOT1	1 (Checked)	Boot from Flash if BOOT0 = 1, otherwise System Memory	This is the factory setting
NRST_MODE	1 or 3	1: Reset input only 3. Bidirectional reset: The NRST pin is configured in reset input/output (legacy) mode	3 is the factory setting

Note: Native USB does not enumerate if HCLK is lower than the default 16 MHz, e.g. by adjusting the AHB prescaler. It will still work in bootloader mode for DFU, it only affects runtime usage of USB e.g. HID, CDC serial.

State	Peripherals	Memory	Voltage Supplies (VDDx)	STM32RTC::LSI_CLOCK	STM32RTC::LSE_CLOCK	STM32RTC::HSE_CLOCK
LowPower.idle()	On	On	On	Supported	Supported	Supported
LowPower.sleep()	On	On	On	Supported	Supported	Supported
LowPower.deepSleep()	On	On	On	Supported	Supported	Supported
LowPower.shutdown()	Off	Off	Off		Supported

: External hardware required

• External 32.768 kHz crystal for LSE²⁾

• rtc.setClockSource(STM32RTC::LSE_CLOCK);
  rtc.begin();

1. Hardware
   1. Choose a 32.768 kHz crystal from AN2867 Guidelines for oscillator design on STM8AF/AL/S and STM32 MCUs/MPUs, §5.2: STM32-compatible low-speed resonators, Table 7: Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products
      • JLCPCB basic part: Seiko Epson FC-135 32.768 KHz 12.5 pF ±20 ppm (I don't recommend this as 12.5pF capacitors require a high drive level and will consume more power during RTC shutdown)
      • A lower g_mcrit value is preferable to reduce the power consumption of the LSE oscillator
      • A higher g_mcrit value will increase power consumption but will have better frequency stability
   2. Take note of the g_mcrit value in the table of the chosen crystal
   3. Choose the load capacitance
      • C1 = C2 = 2 * (C_L - C_stray)
      • “Stray capacitance C_stray comes from the pins of the chip and parasitics from the board. It is often approximated as 5pF.”³⁾
2. Software
   1. Configure LSE drive level: __HAL_RCC_LSEDRIVE_CONFIG(__LSEDRV__) based on the g_mcrit value
      1. RCC_LSEDRIVE_LOW
      2. RCC_LSEDRIVE_MEDIUMLOW (default if __HAL_RCC_LSEDRIVE_CONFIG is not called)
      3. RCC_LSEDRIVE_MEDIUMHIGH
      4. RCC_LSEDRIVE_HIGH
      5. The LSE drive level's G_{m_crit_max} must be higher than the g_mcrit value;
         e.g. if g_mcrit = 2.0, RCC_LSEDRIVE_HIGH must be used, as RCC_LSEDRIVE_MEDIUMHIGH's G_{m_crit_max} of 1.7 was exceeded