There are conflicting things written in STM32 documentation regarding SRAM retention during Standby mode with SRM32 G0 MCU.
For example, in RM0454 G0 Reference Manual it states two different things 1 paragraph apart
.
The UM2319 G0 HAL/LL Manual has a function which states that it controls SRAM retention during standby functionality:
But when I try to compile...
Looking at the contents of `stm32g0xx_hal_pwr_ex.c` I see that the function I'm trying to access is being excluded from compilation by the pre-processor because the RRS bit is not defined:
Looking at the RM0454 G0 Reference Manual again, I see that there is no RRS bit specified in PWR_CR3!
A quick google search seems to indicate it's bit 8 that is defined in some header files out there. These header files seemed to be from stm32g03x1 series controllers.... looking at RM0444 G0x1 Reference Manual the plot thickens:
It would seem the G0x1 "Access" line of micros has this bit defined in its manual but the G0x0 "Value line" data sheet doesn't?
Is that intentional? Is the G0x0 "Value line" microcontroller able to retain its SRAM in standby mode???
Let's test it out!
Added this define to stm32g030xx.h` to get the sram retention HAL function to compile
#define PWR_CR3_RRS_Pos (8U)
#define PWR_CR3_RRS_Msk (0x1UL << PWR_CR3_RRS_Pos)
#define PWR_CR3_RRS PWR_CR3_RRS_MskCreated a test program to just test SRAM rention my toggling it on and off depending on whether or not the previous runtime's SRAM has been retained or not, and to flash an LED on my custom PCBA depending on the result. This is accomplished by creating a `NOINIT` section in my linkerscript and putting the variable sram_magic_word in it.
Basically app does this:
- Flash at 1Hz when SRAM retained when expected
- Flash at 2Hz when SRAM retained when not expected
- Flash at 10Hz when SRAM not retained
- Go into standby mode after 5 seconds
__attribute__((section(".app_noinit"))) uint32_t sram_magic_word;
int main(void)
{
/* USER CODE BEGIN 1 */
/* USER CODE END 1 */
/* MCU Configuration--------------------------------------------------------*/
/* Reset of all peripherals, Initializes the Flash interface and the Systick. */
HAL_Init();
/* USER CODE BEGIN Init */
/* USER CODE END Init */
/* Configure the system clock */
SystemClock_Config();
/* USER CODE BEGIN SysInit */
//We need to configure a pushbutton, an LED (open drain), and a buck converter enable pin
LL_GPIO_SetPinMode(BUTTON_MCU_GPIO_Port, BUTTON_MCU_Pin, LL_GPIO_MODE_INPUT);
LL_GPIO_InitTypeDef GPIO_InitStruct = {
.Pin = LED_3_Pin,
.Mode = LL_GPIO_MODE_OUTPUT,
.Speed = LL_GPIO_SPEED_FREQ_LOW,
.OutputType = LL_GPIO_OUTPUT_OPENDRAIN,
.Pull = LL_GPIO_PULL_NO
};
/**/
LL_GPIO_Init(LED_3_GPIO_Port, &GPIO_InitStruct);
/*Need to configure buck converter to be on in order to to PCBA revision retrieval*/
GPIO_InitStruct.Pin = BUCK_EN_Pin;
GPIO_InitStruct.Mode = LL_GPIO_MODE_OUTPUT;
GPIO_InitStruct.Speed = LL_GPIO_SPEED_FREQ_LOW;
GPIO_InitStruct.OutputType = LL_GPIO_OUTPUT_PUSHPULL;
GPIO_InitStruct.Pull = LL_GPIO_PULL_NO;
LL_GPIO_Init(BUCK_EN_GPIO_Port, &GPIO_InitStruct);
#define TEST_SRAM_STANDBY_RETENTION
#ifdef TEST_SRAM_STANDBY_RETENTION
unsigned int blink_start_ms = uwTick;
//If the bit is indeed controlling SRAM retention, then we would expect that on each button press, we alternate between 1Hz blink for 5 s and 2 Hz blink for 5 s
//If SRAM retained all the time then we would expect button press to always yield 1 Hz blink for 5 s (on second button press onwards)
//If SRAM was retained when expected, blink for 5 seconds at 1 Hz
if(sram_magic_word == 0xDEADBEEF){
sram_magic_word = 0xBADBABEE;
HAL_PWREx_DisableSRAMRetention();
LL_GPIO_SetOutputPin(BUCK_EN_GPIO_Port, BUCK_EN_Pin); //Turn on buck converter to power LED display
while(uwTick - blink_start_ms < 5000){
LL_IWDG_ReloadCounter(IWDG);//Pet watchdog so the MCU doesn't reset on us
LL_GPIO_TogglePin(LED_3_GPIO_Port, LED_3_Pin);
HAL_Delay(500);
};
}
//If SRAM was retained when not expected, blink for 5 seconds at 2 Hz
else if (sram_magic_word == 0xBADBABEE){
sram_magic_word = 0xDEADBEEF;
HAL_PWREx_EnableSRAMRetention();
LL_GPIO_SetOutputPin(BUCK_EN_GPIO_Port, BUCK_EN_Pin); //Turn on buck converter to power LED display
while(uwTick - blink_start_ms < 5000){
LL_IWDG_ReloadCounter(IWDG);//Pet watchdog so the MCU doesn't reset on us
LL_GPIO_TogglePin(LED_3_GPIO_Port, LED_3_Pin);
HAL_Delay(250);
};
}
//If SRAM variable not retained, blink for 5 seconds at 10 Hz
else{
sram_magic_word = 0xDEADBEEF;
HAL_PWREx_EnableSRAMRetention();
LL_GPIO_SetOutputPin(BUCK_EN_GPIO_Port, BUCK_EN_Pin); //Turn on buck converter to power LED display
while(uwTick - blink_start_ms < 5000){
LL_IWDG_ReloadCounter(IWDG);//Pet watchdog so the MCU doesn't reset on us
LL_GPIO_TogglePin(LED_3_GPIO_Port, LED_3_Pin);
HAL_Delay(100);
};
}
//Turn off buck converter
LL_GPIO_ResetOutputPin(BUCK_EN_GPIO_Port, BUCK_EN_Pin);
//Code for entering standby mode
HAL_PWR_EnableWakeUpPin(PWR_WAKEUP_PIN2_HIGH); //Wake up on pushbutton press
LL_PWR_ClearFlag_WU(); //Need to clear this flag else we wakeup immediately upon entry in standby
LL_PWR_ClearFlag_SB(); //Need to clear this flag else we wakeup immediately upon entry in standby
HAL_DBGMCU_DisableDBGStandbyMode(); //Disable debug interface when entering low power mode (to make sure main regulator shuts down?)
HAL_PWR_EnterSTANDBYMode(); //Enter standby mode (see you next runtime!)
#endifNote: SRAM gets retained regardless of whether or not the RRS bit is set when you are debugging. That's part of the reason this test app is written this way; so that you can determine when/how SRAM is getting retained without the use of a debugger.
Using the above program with my STM32G030F6, I was able to confirm that YES INDEED, there is an undocumented RSS bit (bit 😎 in PWR_CR3 register that controls SRAM retention during standby mode for STM32G0x0 Value Line MCU's.
I think we need a new G0x0 Reference Manual with the correct doucmentation on the PWR_CR3 register!!!
