STM32 Nucleo F429ZI UART RS232 receiving false data

[LMüll.1]

I'm trying to read GPS-Data on the F429ZI that is sent from a GPS Module (NMEA GGA & VTG) with 38400 Baud but I only receive weird symbols. I tried sending specific values from my PC but they also turn into nonsense on the receive-side.

"U" (85dec / 1010101) is the only symbol that is received correctly. But when I try to send it back to the PC, it also changes value.

Equal values sent produce the same wrong value on the uC but I cant see a relation between sent and received values.

Some Examples:

255dec -> 0

0 -> 0

177dec ->69dec

65dec -> 95dec

I read a lot about these problems that were solved with a change in clock frequencies but cant't make it work for me with any frequency combination.

Thanks for your help!

(I modified a STM UART-Example for my purpose.)

main.h

/* Define to prevent recursive inclusion -------------------------------------*/
#ifndef __MAIN_H__
#define __MAIN_H__
 
/* User can use this section to tailor USARTx/UARTx instance used and associated
   resources */
   /* Definition for USARTx clock resources */
#define USARTx                           USART3
#define USARTx_CLK_ENABLE()              __HAL_RCC_USART3_CLK_ENABLE()
#define USARTx_RX_GPIO_CLK_ENABLE()      __HAL_RCC_GPIOB_CLK_ENABLE()
#define USARTx_TX_GPIO_CLK_ENABLE()      __HAL_RCC_GPIOB_CLK_ENABLE()
 
#define USARTx_FORCE_RESET()             __HAL_RCC_USART3_FORCE_RESET()
#define USARTx_RELEASE_RESET()           __HAL_RCC_USART3_RELEASE_RESET()
 
/* Definition for USARTx Pins */
#define USARTx_TX_PIN                    GPIO_PIN_10
#define USARTx_TX_GPIO_PORT              GPIOB
#define USARTx_TX_AF                     GPIO_AF7_USART3
#define USARTx_RX_PIN                    GPIO_PIN_11
#define USARTx_RX_GPIO_PORT              GPIOB
#define USARTx_RX_AF                     GPIO_AF7_USART3
 
 
#define USER_Btn_Pin GPIO_PIN_13
#define USER_Btn_GPIO_Port GPIOC
#define LD3_Pin GPIO_PIN_14
#define LD3_GPIO_Port GPIOB
#define LD2_Pin GPIO_PIN_7
#define LD2_GPIO_Port GPIOB
 
/* ########################## Assert Selection ############################## */
/**
  * @brief Uncomment the line below to expanse the "assert_param" macro in the 
  *        HAL drivers code
  */
/* #define USE_FULL_ASSERT    1U */
 
#ifdef __cplusplus
 extern "C" {
#endif
void _Error_Handler(char *, int);
 
#define Error_Handler() _Error_Handler(__FILE__, __LINE__)
#ifdef __cplusplus
}
#endif
 
#endif /* __MAIN_H__ */

main.c

UART_HandleTypeDef UartHandle;
char RxByte;
 
 
void SystemClock_Config(void);
 
void USART3_IRQHandler(void)
{
	HAL_UART_IRQHandler(&UartHandle);
}
 
void HAL_UART_RxCpltCallback(UART_HandleTypeDef* huart)
{
	if (huart->Instance == USART3)
	{
		/* Receive one byte in interrupt mode */
		HAL_UART_Receive_IT(&UartHandle, &RxByte, 1);
		HAL_UART_Transmit_IT(&UartHandle, &RxByte, 1);
	}
}
 
int main(void)
{
	/* Reset of all peripherals, Initializes the Flash interface and the Systick. */
	HAL_Init();
 
	/* Configure the system clock */
	SystemClock_Config();
 
	/*##-1- Configure the UART peripheral ######################################*/
    /* Put the USART peripheral in the Asynchronous mode (UART Mode) */
    /* UART configured as follows:
	- Word Length = 8 Bits (7 data bit + 1 parity bit) :
					BE CAREFUL : Program 7 data bits + 1 parity bit in PC HyperTerminal
	- Stop Bit    = One Stop bit
	- Parity      = ODD parity
	- BaudRate    = 9600 baud
	- Hardware flow control disabled (RTS and CTS signals) */
 
	UartHandle.Instance = USARTx;
	UartHandle.Init.BaudRate = 38400;
	UartHandle.Init.WordLength = UART_WORDLENGTH_9B;
	UartHandle.Init.StopBits = UART_STOPBITS_1;
	UartHandle.Init.Parity = UART_PARITY_NONE;
	UartHandle.Init.HwFlowCtl = UART_HWCONTROL_NONE;
	UartHandle.Init.Mode = UART_MODE_TX_RX;
	UartHandle.Init.OverSampling = UART_OVERSAMPLING_16;
	if (HAL_UART_Init(&UartHandle) != HAL_OK)
	{
		/* Initialization Error */
		Error_Handler();
	}
 
	HAL_UART_MspInit(&UartHandle);
 
	/* Peripheral interrupt init*/
	HAL_NVIC_SetPriority(USART3_IRQn, 0, 0);
	HAL_NVIC_EnableIRQ(USART3_IRQn);
 
	if (HAL_UART_Receive_IT(&UartHandle, &RxByte, 1) != HAL_OK)
	{
		HAL_GPIO_WritePin(LD2_GPIO_Port, LD2_Pin, 1);
	}
 
	/* Infinite loop */
	while (1)
	{
		
	}
 
}
 
 
void SystemClock_Config(void) 
{
	RCC_OscInitTypeDef RCC_OscInitStruct;
	RCC_ClkInitTypeDef RCC_ClkInitStruct;
 
	/**Configure the main internal regulator output voltage
	*/
	__HAL_RCC_PWR_CLK_ENABLE();
 
	__HAL_PWR_VOLTAGESCALING_CONFIG(PWR_REGULATOR_VOLTAGE_SCALE3);
 
	/**Initializes the CPU, AHB and APB busses clocks
	*/
	RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
	RCC_OscInitStruct.HSIState = RCC_HSI_ON;
	RCC_OscInitStruct.HSICalibrationValue = 16; 
	RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
	RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;
	RCC_OscInitStruct.PLL.PLLM = 16;
	RCC_OscInitStruct.PLL.PLLN = 72;
	RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV2; 
	RCC_OscInitStruct.PLL.PLLQ = 7;
	if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
	{
		_Error_Handler(__FILE__, __LINE__);
	}
 
	/**Initializes the CPU, AHB and APB busses clocks
	*/
	RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK | RCC_CLOCKTYPE_SYSCLK
		| RCC_CLOCKTYPE_PCLK1 | RCC_CLOCKTYPE_PCLK2;
	RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
	RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
	RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV2;
	RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV2;
 
	if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_2) != HAL_OK)
	{
		_Error_Handler(__FILE__, __LINE__);
	}
 
	/**Configure the Systick interrupt time
	*/
	HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq() / 1000);
 
	/**Configure the Systick
	*/
	HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK);
 
	/* SysTick_IRQn interrupt configuration */
	HAL_NVIC_SetPriority(SysTick_IRQn, 0, 0);
}
 

system_stm32f4xx.c (snippet)

 
#include "stm32f4xx.h"
 
#if !defined  (HSE_VALUE) 
  #define HSE_VALUE    ((uint32_t)8000000) /*!< Default value of the External oscillator in Hz */ 
#endif /* HSE_VALUE */
 
#if !defined  (HSI_VALUE)
  #define HSI_VALUE    ((uint32_t)16000000) /*!< Value of the Internal oscillator in Hz*/ //DEFAULT 16000000
#endif /* HSI_VALUE */
 

stm32f4xx_hal_conf.h (snippet)

/* ########################## HSE/HSI Values adaptation ##################### */
/**
  * @brief Adjust the value of External High Speed oscillator (HSE) used in your application.
  *        This value is used by the RCC HAL module to compute the system frequency
  *        (when HSE is used as system clock source, directly or through the PLL).  
  */
#if !defined  (HSE_VALUE) 
  #define HSE_VALUE    ((uint32_t)8000000U) /*!< Value of the External oscillator in Hz */ //TEST 25000000) //DEFAULT: 8000000U
#endif /* HSE_VALUE */
 
#if !defined  (HSE_STARTUP_TIMEOUT)
  #define HSE_STARTUP_TIMEOUT    ((uint32_t)100U)   /*!< Time out for HSE start up, in ms */
#endif /* HSE_STARTUP_TIMEOUT */
 
/**
  * @brief Internal High Speed oscillator (HSI) value.
  *        This value is used by the RCC HAL module to compute the system frequency
  *        (when HSI is used as system clock source, directly or through the PLL). 
  */
#if !defined  (HSI_VALUE)
  #define HSI_VALUE    ((uint32_t)16000000U) /*!< Value of the Internal oscillator in Hz*/ //DEFAULT: 16000000U
#endif /* HSI_VALUE */
 

[Tesla DeLorean]

The STM32 expects CMOS levels, not RS232 ones. Use a MAX3232 or equivalent.

Get a scope and check the baud rate by looking at the bit timing. Most GPS default to 9600 baud.

[LMüll.1]

Thank you for your quick response!

The baudrate is stated in the datasheet of the GPS so it should be correct but I'll try to get a RS232 - CMOS Converter

Thanks a lot!