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--- projects:electronics:stm32:stm32u0_notes [2025/03/12 16:22] – Lots more literature and information Andrew Yong
+++ projects:electronics:stm32:stm32u0_notes [2025/03/30 09:33] (current) – [Choosing an LSE Crystal] redundant "substitute" typo Andrew Yong
@@ Line 33: / Line 33: @@
     * 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 Reset((RM0503, §3.7.7 FLASH option register (FLASH_OPTR), Bits 10:8 BOR_LEV[2:0]: BOR reset level))
+  * Brownout Reset((RM0503, §3.7.7: FLASH option register (FLASH_OPTR), Bits 10:8 BOR_LEV[2:0]: BOR reset level))
     * Selectable: 1.7, 2.0, 2.2, 2.5 or 2.8 V
 ===== Native USB (STM32U073, STM32U083) =====
-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.
+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 ^
@@ Line 45: / Line 45: @@
 | 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.
 ===== STM32duino LowPower & RTC =====
@@ Line 57: / Line 58: @@
 ==== Prerequisites for LowPower.shutdown() ====
-  * External 32.768 kHz crystal for LSE((DS14581/DS14548/DS14463, §6.3.7 External clock source characteristics, Low-speed external clock generated from a crystal resonator))
+  * External 32.768 kHz crystal for LSE((DS14581/DS14548/DS14463, §6.3.7: External clock source characteristics, Low-speed external clock generated from a crystal resonator))
   * <code>rtc.setClockSource(STM32RTC::LSE_CLOCK);
 rtc.begin();</code>
-==== RTC Clock Sources ====
+===== Choosing an LSE Crystal =====
+  - **Hardware**
+    - Choose a **32.768 kHz crystal oscillator**(([[https://www.st.com/resource/en/application_note/an2867-guidelines-for-oscillator-design-on-stm8afals-and-stm32-mcusmpus-stmicroelectronics.pdf|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: [[https://jlcpcb.com/partdetail/SeikoEpson-Q13FC13500004/C32346|Seiko Epson FC-135 32.768 KHz 12.5 pF ±20 ppm]]
+    - Choose the **load capacitance**
+      * C1 = C2 = 2 * (C<sub>L</sub> - C<sub>stray</sub>)
+      * "Stray capacitance C<sub>stray</sub> comes from the pins of the chip and parasitics from the board. It is often approximated as 5pF."(([[https://microchip.my.site.com/s/article/Calculating-crystal-load-capacitor|Calculating crystal load capacitor]]))
+  - **Software**
+    - Configure **LSE drive level**: ''%%__HAL_RCC_LSEDRIVE_CONFIG%%(//%%__LSEDRV__%%//)''; Select the lowest value that can provide robust LSE start-up((RM0503, §5.2.6: LSE clock))\\ Substitute ''//%%__LSEDRV__%%//'' with:
+      - ''RCC_LSEDRIVE_LOW''
+      - ''RCC_LSEDRIVE_MEDIUMLOW'' (default if ''%%__HAL_RCC_LSEDRIVE_CONFIG%%'' is not called)
+      - ''RCC_LSEDRIVE_MEDIUMHIGH''
+      - ''RCC_LSEDRIVE_HIGH''

ndoo.sg

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