STM32 UART does not receive first byte

[HamzaBeder]

MCU: STM32L476RGT6

Hello,

I am using UART to read data from the NEO-M9N GPS receiver. My receive buffer stores all received data besides the first byte:

This forum states to add a delay between initializing UART and sending data, and this forum states to clear the buffer before transmitting data. However, neither solution works for me. These forums are addressing an issue where the first byte received is 0x00. However, my issue is that rather than receive 0x00, I skip the first byte entirely.

Any tips on how to fix this would be appreciated. Thanks.

static void MX_USART3_UART_Init(void);
UART_HandleTypeDef huart3;
void SystemClock_Config(void);
static void MX_GPIO_Init(void);
uint8_t buffer[100];
uint8_t buffer_idx;

int main(void)
{
 HAL_Init();
 SystemClock_Config();
 MX_GPIO_Init();
 MX_USART3_UART_Init();
 buffer_idx=0;
 while(HAL_TIMEOUT != HAL_UART_Receive(&huart3, buffer, 1, 10));
 HAL_UART_Receive_IT(&huart3, buffer, 1);
 HAL_Delay(1);
 while (1) {
  uint8_t arr[] = {0xB5, 0x62, 0x06, 0x08, 0x00, 0x00, 0x0E, 0x30};
  HAL_UART_Transmit(&huart3, arr, sizeof(arr), HAL_MAX_DELAY);
  HAL_Delay(100);
  buffer_idx=0;
  HAL_Delay(1000);
  }
}

void HAL_UART_RxCpltCallback(UART_HandleTypeDef *huart)
{
 HAL_UART_Receive_IT(&huart3, buffer+buffer_idx, 1);
 buffer_idx++;
}

[nicolas]

You set buffer_idx to 0 but the callback of last byte of the previous reception already called HAL_UART_Receive_IT() with a high value for buffer_idx. Only after the callback of the first rx byte, the HAL_UART_Receive_IT() will use a buffer_idx of 0.

[HamzaBeder]

Sorry, but I am not sure what the "previous reception" is. The first reception should be the byte 0xB5, so there should not be a previous reception that comes before this.

You said that buffer_idx is initially some high value before the "previous reception" and gets set to 0 only after the callback of the first rx byte (0xB5). Why is that? I set buffer_idx to 0 before calling HAL_UART_Receive_IT().

I placed a breakpoint inside my callback function, and when looking inside the RDR register i see 0x62 instead of 0xB5. The callback function is never called when the MCU receives 0xB5, so 0xB5 is never stored inside the buffer. Why does it skip over this first byte?