Verifying if HSE is used by MCU as system clock without probing

Question

I have a PCB where I use an 8 MHz quartz resonator as this MCU's external system clock source.

In STM32CubeIDE the setting is as follows:

The program runs fine in MCU, but I cannot be sure if HSE is really used instead of HSI. Here is the clock config:

As you can see, both HSE and HSI are active. I supposed to be using HSE, but I remember once I tried to set like this in a development board which has no external oscillator and MCU still worked using its HSI.

My worry is that, imagine the crystal is not working, then the MCU will switch HSI without telling me and I will never know. I want to be sure that the system is using the external crystal.

Now the problematic part is that I forgot to add test pins and MCO output is unused and is not routed to any PCB header. Meaning that I'm NOT able to probe the MCU pins.

Is there a way/trick I can try without probing pins physically programmatically that ensures that MCU is using HSE but not HSI? I think it's impossible to disable HSI(?) but can there be any way I can try?

Answer 1

The clocks/pll are configured/selected and can be monitored by the Clock Configuration register and the Clock Control register

Bit 15, 16 in RCC_CFGR register will indicate as well as bit 16,17 in RCC_CR register.

I don't use HAL so I can't help with a function that reads registers.

Answer 2

There are several ways.

The best would be to select HSE as MCO output to verify the output is 8 MHz.

And then probe the crystal with a second scope probe to see that the two waveforms are a match, i.e. same constant phase offset and freqency.

But those cannot be done without probing.

The generated MCU startup code will usually hang in a loop if you try to start HSE because it does not become ready and oscillate.

Also if you set the parameters for the PLL to expect 8 MHz clock from HSE, please note that you cannot select the 8 MHz HSI as PLL clock source, the PLL can only get prescaled 4 MHz from PLL.

So if you e.g. set 8MHz HSE as PLL source and set the PLL dividers and multipliers so that it makes a 72 MHz clock from 8 MHz, and blink a LED at 1 Hz, the CPU will run at 72 MHz and blink the LED at 1 MHz, or, if accidentally the 4 MHz from HSI is selected as PLL source, the CPU will run at half speed of 36 MHz and the LED will blink at half speed of 2 Hz.

So as the CPU cannot run at 72 MHz unless the PLL input is from HSE, 8 MHz multiplied by 9. With HSI, the maximum achievable clock is only 64 MHz, 4 MHz multiplied by 16x, but if the multiplier is set to 9, with HSI that is 36 MHz.

The code can also enable the Clock Security System and you get an interrupt that HSE failed to work if it was in use.