Wrong SysTick prescaler [CUBEMX]

[SZano]

In CubeMX, in the clock tree screen, you can select the prescaler that produces "Cortex System Timer (MHz)" (which is the clock feeding SysTick) starting from HCLK.

However, whether you select the /1 prescaler or the /8 prescaler, identical code is generated (see the attachment).

I would expect this setting to be reflected in some initialization code.

The SysTick->CTRL register has a bit (bit #2, defined as SysTick_CTRL_CLKSOURCE_Msk in "core_cm*.h") to select the SysTick clock source.

From the ARM documentation (https://developer.arm.com/documentation/dui0497/a/cortex-m0-peripherals/optional-system-timer--systick/systick-control-and-status-register) this bit selects either a "reference clock" (if 0) or a "processor clock" (if 1).

CubeMX-generated code always sets this bit to 1.

The reference manual (for L073) says, in the RCC chapter:

The RCC feeds the Cortex® System Timer (SysTick) external clock with the AHB clock

(HCLK) divided by 8. The SysTick can work either with this clock or with the Cortex® clock

(HCLK), configurable in the SysTick Control and Status Register.

I've tried changing the CubeMX setting both on an STM32L073 and on an STM32F746; in both cases, the code disregards the prescaler settings.

The issue is that "core_cm*.h" has SysTick_CTRL_CLKSOURCE_Msk hardcoded (in function SysTick_Config()), and this function is probably untouchable, being part of CMSIS.

So, HAL libraries and CubeMX code generation will have to ignore this function, and reimplement it in HAL_SYSTICK_Config() with the proper behavior, by setting SysTick->CTRL to the correct value.

Alternatively, the CubeMX clock tree screen should show a clear warning when selecting "/8", about it not being supported by code generation and about the need to manually tweak SysTick_CTRL_CLKSOURCE.