Hi all:
I have a general question about whether STM32 peripherals need to be clocked permanently.
I am using an STM32G0 at the moment, and I am disabling the USB C Power Delivery "Dead Battery" feature, in order to make sure that the internal 5k1 pull-downs on the UCPDx_CCx pins (PA8, PB15, PD0, PD2) are disabled:
LL_SYSCFG_DisableDBATT( LL_SYSCFG_UCPD1_STROBE | LL_SYSCFG_UCPD2_STROBE );
Nothing indicates that you need to clock any peripheral to do that, but LL_SYSCFG_DisableDBATT() writes to SYSCFG->CFGR1, so I am guessing that you need to clock SYSCFG:
LL_APB2_GRP1_EnableClock( LL_APB2_GRP1_PERIPH_SYSCFG );
However, other people also clock PWR. I may be because other families STM32G4 and STM32L5 disable those pull-down resistors in a different way:
PWR->CR3 |= PWR_CR3_UCPD_DBDIS; // See LL_PWR_DisableUCPDDeadBattery().
So I wonder whether clocking PWR is necessary on the STM32G0 to do that task.
The code which STM32CubeMX generates for the STM32G0 does clock PWR too, probably because it calls HAL_PWREx_ControlVoltageScaling().
However, the voltage scaling upon reset in PWR_CR1 ist already the right level "VOS = 01: Range 1 (high-performance range)" for the maximum 64 Mhz operation, so there is no need to set it at all.
The HAL code waits until the desired power level has been reached:
while (HAL_IS_BIT_SET(PWR->SR2, PWR_SR2_VOSF))
But I wonder whether that is necessary. I would be silly that the STM32G0 starts configured with the high-performance range, but before its power regulator has actually reached the proper level. I could not find that detail in the documentation.
After all these ramblings, I am also wondering whether such peripherals need to be clocked permanently. Reference manual RM0444 states:
"When the clock to a peripheral or I/O port is not active, the read and write accesses to its registers are not effective."
I would expect the registers to read as 0 if the peripheral is not clocked then.
However, reading PWR->CR1 when PWR is not clocked does not return 0, but a plausible value, at least on the STM32G0.
Say I just need to check some register values, or to set some options, only once on start-up. Can I clock those peripherals, read and/or write the affected bits, and then stop the clock? Or would they stop functioning properly?
Obviously, a USART or an SPI need a clock to operate, but does SYSCFG or PWR?
Let's change now to the STM32F4 family. Its Power Controller (PWR) has a Power Supply Supervisor (for brown-out reset). If configure it, and then stop the PWR clock, will the configured brown-out reset stop working?
Thanks in advance,
rdiez
