Runtime Clock Frequency Change from 168MHz to 84MHz and viceversa on stm32h743zi and stm32u575zi_q

[NamrataGagare-eaton]

Background:
I'm working on an embedded application using STM32H743ZI and STM32U575ZI_Q with Zephyr RTOS v3.7.1. Currently, I have a device tree overlay that configures the system clock at 168MHz, but I need the ability to dynamically switch to 84MHz at runtime.

Current Configuration:
My current device tree overlay (nucleo_h743zi.overlay) has:

/* Active 168MHz Configuration */
&pll {
div-m = <1>;
mul-n = <42>;
div-p = <2>;
div-q = <6>;
div-r = <2>;
clocks = <&clk_hse>;
status = "okay";
};

&rcc {
clocks = <&pll>;
clock-frequency = <DT_FREQ_M(168)>;
d1cpre = <1>;
hpre = <1>;
d1ppre = <2>;
d2ppre1 = <2>;
d2ppre2 = <2>;
d3ppre = <2>;
};

&timers2 {
st,prescaler = <239>;
counter {
compatible = "st,stm32-counter";
status = "okay";
};
};

/* Desired 84MHz Configuration (currently commented) */
&pll {
div-m = <1>;
mul-n = <21>;
div-p = <2>;
div-q = <3>;
div-r = <2>;
clocks = <&clk_hse>;
status = "okay";
};

&rcc {
clocks = <&pll>;
clock-frequency = <DT_FREQ_M(84)>;
d1cpre = <1>;
hpre = <1>;
d1ppre = <2>;
d2ppre1 = <2>;
d2ppre2 = <2>;
d3ppre = <2>;
};

&timers2 {
st,prescaler = <1>;
counter {
compatible = "st,stm32-counter";
status = "okay";
};
};

Primary Questions:

1. Is runtime clock switching supported in Zephyr?

• Can I dynamically change the system clock frequency after boot?
• Are there existing Zephyr APIs for this purpose?
• What are the limitations and considerations?

2. Recommended approach for STM32?

• Should I use direct STM32 HAL calls within Zephyr?
• Are there Zephyr clock management APIs I should use instead?
• How do I handle peripheral clock dependencies?

3. Safe implementation strategy?

• What precautions are needed when changing clocks at runtime?
• How do I ensure system stability during clock transitions?
• Which peripherals need to be stopped/reconfigured?

Any guidance, code examples, or pointers to relevant documentation would be greatly appreciated!

Thanks,
Namrata Gagare