UART not responding under OSC BYPASS Mode.

[PKuma.51]

Hello Guys,

I am trying to setup UART under oscillator bypass mode in STM32f446RE Nucleo Board. I have created a new project in CubeIDE with UART under Asynchronous mode. I have commented out the SystemClock_Config and written my code for oscillator bypass mode even though everything can be done via code generation tool ( Part of learning ). But UART is not responding. Please find the below-attached project.

char data[] = "PK is Working\r\n";
 
int main(void)
{
  HAL_Init();
  //SystemClock_Config();
 
  RCC_OscInitTypeDef Osc = {0};
  RCC_ClkInitTypeDef Clk = {0};
 
  Osc.OscillatorType = RCC_OSCILLATORTYPE_HSE;
  Osc.HSEState = RCC_HSE_BYPASS;
  HAL_RCC_OscConfig(&Osc);
 
  Clk.ClockType = RCC_CLOCKTYPE_SYSCLK | RCC_CLOCKTYPE_HCLK | RCC_CLOCKTYPE_PCLK1 | RCC_CLOCKTYPE_PCLK2;
  Clk.AHBCLKDivider = RCC_SYSCLK_DIV2;
  Clk.SYSCLKSource = RCC_SYSCLKSOURCE_HSE;
  Clk.APB1CLKDivider = RCC_HCLK_DIV2;
  Clk.APB2CLKDivider = RCC_HCLK_DIV2;
 
  HAL_RCC_ClockConfig(&Clk, FLASH_ACR_LATENCY_0WS);
 
  __HAL_RCC_HSI_DISABLE();
 
  HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq()/1000);
  HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK);
 
 
 
  MX_GPIO_Init();
  MX_USART2_UART_Init();
 
  while (1)
  {
   HAL_UART_Transmit(&huart2, (uint8_t*) data, sizeof(data), HAL_MAX_DELAY);
  }
}

[TDK]

Debug your code. Do all the calls to HAL_* return HAL_OK? Have you observed the TX line with a scope to verify it's not responding?

Your clock is pretty slow. Is your UART baud rate within tolerance?

[PKuma.51]

After some debugging, I found that when the function HAL_RCC_ClockConfig(&Clk, FLASH_ACR_LATENCY_0WS) exit, the value of SystemCoreClock changes from 16000000 to 12500000. I am expecting an HCLK of 4Mhz. The UART sometimes respond with random garbage values. I dont understand why this happens? Any solution guys?

[waclawek.jan]

You need to set properly the constant which determines the HSE frequency - you've copied some project based on a board which has 25MHz crystal at HSE.

I don't know which constant it is, I don't use Cube, but Clive mentioned it here many times in the past in similar cases.

JW

[TDK]

As waclawek.jan says, change HSE_VALUE to 8000000.