STM32F411 PLL Problem ("Target is not responding, retrying...")

[Renier]

Hi, 

I am experiencing a problem on the STM32F411RET6 and STM32F411RCT6 MCUs. When programming these devices I am getting a repeated "Target is not responding, retrying..." message until the debugging session shuts down and exits. I have managed to narrow it down to the something related to the Main PLL. The MCU works if the 16MHz HSI RC clock or the 8 MHz HSE clock is selected as the System Clock Mux source, but fails with the "Target is not responding, retrying..." message if the PLLCLK is selected irrespective of whether the HSI or HSE is selected as the PLL Source.

In a desperate effort I've also replaced the STM32F411 with a STM32L433RCT6 which pins are almost an exact match to that of the STM32F411. Using this MCU the PLL works as expected and I can use either the HSI or HSE as the PLL Source.

I've attached the non-working STM32CubeIDE STM32F411 project which toggles pin PC8. Any assistance will be greatly appreciated. 

Thanks

Renier

[Tesla DeLorean]

Your own board?

Check the VCAP's placed,  one-up 4u7 F, or two-up 2u2 F, voltage should be around 1.25V

Other than that watch VOS configuration, HSE frequency, and PLL VCO frequency not exceeding design limits.

[Renier · Accepted solution]

Hi, 

I managed to solve the problem. The VREF+ voltage was incorrect set at 2.1V which means that the condition of  VDDA-VREF+ < 1.2 V was not being met (Note 3 on page 64 of the Data Sheet). After correcting this issue the main PLL works perfectly. 

Thanks

Renier