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[STM32L4] [PWR] When MCU Vdd is given 1.8V supply, the LPUART and USB do not work

Question asked by bhayani.sunny on Aug 19, 2016
Latest reply on Aug 19, 2016 by FTITI.Walid

I am using STM32L4 - Vdd @3V (at 48MHz and 80MHz). For both the clock frequencies, i.e. 48MHz and 80MHz, I am able to use the LPUART1 and USB (Device Mode) properly.

My RCC configuration is:

     LPUART1 LSE Oscillator and LSE Clock.
     MSI Clock Range - MSI Range 6 (Voltage Scaling @ Run Range 1)
     PLL Source - MSI
     PLLM - 1
     PLLN - 24
     PLLP - 7
     PLLQ - 2
     PLLR - 2
     SYSCLKSource - PLLCLK
     AHB CLK Div - 1 (SYSCLK)
     APB1 and APB2CLKDiv - 1 (HCLK)
     USBCLKSource - PLL
But when the Vdd is changed to 1.8V (SDCard at 3V), the LPUART1 and USB (Device Mode), does not work. The code is executing properly, but I do not see any data being sent out from the LPUART1. Similarly, I do not get the USB Interrupts.

I referred the PWR chapter in the Reference Manual (RM0351), but did not find anything which mentions that the LPUART1 and USB will stop working when the Vdd is at 1.8V.

One more observation is that, when the Vdd is @1.8V, the LPUART1 works till the SYSCLK is configured at 32MHz. But it does not work when the SYSCLK is configured to 48MHz which is required for the USB to work.

Awaiting your reply.