USB is less stable when enabling CRS Automatic Trimming Mode [STM32F072]

[xpp07]

For some reason, when I enable CRS Automatic Trimming my USB connection gets unstable. I need to unplug/plug back the USB several times for the PC to recognize the device. When I disable the Automatic Trimming mode and use the default HSI48CalibrationValue = 0x20, the connectivity improves a lot. See below the clock initialization. I appreciate any input.

void Clocks_Init(void)
{
 static RCC_CRSInitTypeDef RCC_CRSInitStruct;
 
 // CLOCK CONFIGURATION REGISTERS
 RCC->CFGR = RCC_CFGR_SW_HSI; // Confgure SYSCLK 
 RCC->CFGR |= RCC_CFGR_INIT; // Confgure PLL clk 
 
 // CLOCK CONTROL REGISTERS 
 RCC->CR2 |= RCC_CR2_INIT; // Enable HSI14 CLK 
 
 // AHB PERIPHERAL CLOCK ENABLE REGISTERS
 RCC->AHBENR |= RCC_AHBENR_INIT; // Enable DMA, CRC, and GPIO Clks 
 
 // APB PERIPHERAL CLOCK ENABLE REGISTERS 1
 RCC->APB1ENR |= RCC_APB1ENR_INIT; // Enable CRS, WWDOG, & TIM3/2 CLKs 
 
 // APB PERIPHERAL CLOCK ENABLE REGISTERS 2
 RCC->APB2ENR |= RCC_APB2ENR_INIT; // Enable ADC interface and SPI Clks 
 RCC->APB2ENR |= RCC_APB2ENR_DBGMCUEN; // Enable MCU Debug Clk 
 
 CRS->CR |= CRS_CR_AUTOTRIMEN; // Enable CRS Automatic Trimming Mode
 
 /*Configure the clock recovery system (CRS)**********************************/
 
 /*Enable CRS Clock*/
 __HAL_RCC_CRS_CLK_ENABLE(); 
 
 /* Default Synchro Signal division factor (not divided) */
 RCC_CRSInitStruct.Prescaler = RCC_CRS_SYNC_DIV1;
 
 /* Set the SYNCSRC[1:0] bits according to CRS_Source value */
 RCC_CRSInitStruct.Source = RCC_CRS_SYNC_SOURCE_USB;
 
 /* HSI48 is synchronized with USB SOF at 1KHz rate */
 RCC_CRSInitStruct.ReloadValue = __HAL_RCC_CRS_RELOADVALUE_CALCULATE(48000000, 1000);
 RCC_CRSInitStruct.ErrorLimitValue = RCC_CRS_ERRORLIMIT_DEFAULT;
 
 /* Set the TRIM[5:0] to the default value*/
 RCC_CRSInitStruct.HSI48CalibrationValue = 0x20; 
 
 /* Start automatic synchronization */ 
 HAL_RCCEx_CRSConfig (&RCC_CRSInitStruct); 
}

[TDK]

> CRS->CR |= CRS_CR_AUTOTRIMEN; // Enable CRS Automatic Trimming Mode

> __HAL_RCC_CRS_CLK_ENABLE();

You need to enable the clock before modifying registers.

Read out and report the CRS registers after initializing and enabling it.

[xpp07]

Thank you. Although I think the Auto trimming is enabled again in the function HAL_RCCEx_CRSConfig (&RCC_CRSInitStruct);

.......................
/* Adjust HSI48 oscillator smooth trimming */
 /* Set the TRIM[5:0] bits according to RCC_CRS_HSI48CalibrationValue value */
 MODIFY_REG(CRS->CR, CRS_CR_TRIM, (pInit->HSI48CalibrationValue << CRS_CR_TRIM_BITNUMBER));
 
 /* START AUTOMATIC SYNCHRONIZATION*/
 
 /* Enable Automatic trimming & Frequency error counter */
 SET_BIT(CRS->CR, CRS_CR_AUTOTRIMEN | CRS_CR_CEN);