doc: release-notes 3.3: Document STM32 USB clock related changes
Document recent changes about USB clock source configuration.
@@ -418,6 +418,18 @@ Drivers and Sensors
* USB
+ * STM32F1: Clock bus configuration is not done automatically by driver anymore.
+ It is user's responsibility to configure the proper bus prescaler using clock_control
+ device tree node to achieve a 48MHz bus clock. Note that, in most cases, core clock
+ is 72MHz and default prescaler configuration is set to achieve 48MHz USB bus clock.
+ Prescaler only needs to be configured manually when core clock is already 48MHz.
+
+ * STM32 (non F1): Clock bus configuration is now expected to be done in device tree
+ using ``clocks`` node property. When a dedicated HSI 48MHz clock is available on target,
+ is it configured by default as the USB bus clock, but user has the ability to select
+ another 48MHz clock source. When no HSI48 is available, a specific 48MHz bus clock
+ source should be configured by user.
+
* W1
* Watchdog
