{"id":862,"date":"2024-07-15T13:12:28","date_gmt":"2024-07-15T11:12:28","guid":{"rendered":"https:\/\/redero.fr\/?p=862"},"modified":"2025-09-19T21:53:52","modified_gmt":"2025-09-19T19:53:52","slug":"clock-tree-configuration-illustrated-with-cubeide-and-zephyr","status":"publish","type":"post","link":"https:\/\/redero.fr\/?p=862&lang=en","title":{"rendered":"Clock Tree Configuration illustrated with CubeIDE and Zephyr"},"content":{"rendered":"\n

Clock configuration is scary: if it doesn\u2019t work, nothing does, and debugging is not easy. Also, it is very architecture dependent, so there is no one clock tree to rule them all.<\/p>\n\n\n\n

STM32 has a nice clock configuration IDE, making it very easy to select the right clock options, so I chose it as an example to show some Zephyr clock tree implementation.<\/p>\n\n\n\n\n\n

Overview<\/h2>\n\n\n\n

In the Nucleo-F756ZG, the external clock is 8 MHz. Check out the schematics<\/a> for this NDK NX3225GD-8.0000M crystal ref:<\/p>\n\n\n\n

\"\"\n\t\t\t\n\t\t\t\t\n\t\t\t<\/svg>\n\t\t<\/button><\/figure>\n\n\n\n

This clock input is also named HSE: High-Speed External oscillator.<\/p>\n\n\n\n

If no external clock was present, we could use HSI (High-Speed Internal oscillator), generated in the chip itself, it is less accurate but it saves you an external component.<\/p>\n\n\n\n

We also need a Low-Speed clock for low-power stuff, and since there is no 32.768 KHz external clock onboard (aka LSE), we have to use the internal one (aka LSI).<\/p>\n\n\n\n

Let\u2019s open STM32CubeIDE Clock Configuration GUI tool:<\/p>\n\n\n\n

\"\"\n\t\t\t\n\t\t\t\t\n\t\t\t<\/svg>\n\t\t<\/button><\/figure>\n\n\n\n

This interface shows the whole clock tree system for the SoC. On the left-hand side, we can see the HSE and LSE external inputs, and how they are muxed with HSI and LSI.<\/p>\n\n\n\n

Clock input<\/h2>\n\n\n\n

Here is the Zephyr device tree description for a high-speed 8MHz external crystal:<\/p>\n\n\n\n

clk_hse: clk-hse {\n        #clock-cells = <0x0>;\n        compatible = \"st,stm32-hse-clock\";\n        status = \"okay\";\n        hse-bypass;\n        clock-frequency = <DT_FREQ_M(8)>;\n};<\/code><\/pre>\n\n\n\n
.\/boards\/st\/nucleo_f756zg\/nucleo_f756zg.dts<\/p>\n\n\n\n
\u24d8 hse-bypass means you are bypassing HSI with HSE, not that you are bypassing HSE. Device tree bindings description states:<\/p>\n\n\n\n
hse-bypass: HSE crystal oscillator bypass
Set to the property to by-pass the oscillator with an external clock.<\/code><\/pre>\n\n\n\n
PLL configuration<\/h2>\n\n\n\n
The PLL (Phased-Locked Loop) is a hardware thingy allowing to convert one frequency to another. There is a pipeline of fixed-values multiply and divide operations, so you have to play around with maths to get to the values you want. Here we end up with 216MHz derived from the 8MHz input.<\/p>\n\n\n\n
\"\"\n\t\t\t\n\t\t\t\t\n\t\t\t<\/svg>\n\t\t<\/button><\/figure>\n\n\n\n
pll: pll {\n       #clock-cells = <0x0>;\n       compatible = \"st,stm32f7-pll-clock\";\n       status = \"okay\";\n       div-m = <4>;\n       mul-n = <216>;\n       div-p = <2>;\n       div-q = <9>;\n       clocks = <&clk_hse>;\n};<\/code><\/pre>\n\n\n\n
Compiled .\/build\/zephyr\/zephyr.dts<\/p>\n\n\n\n
If the CubeIDE way is not your way, it is possible to use pen and paper and calculator.<\/p>\n\n\n\n