Why "HAL_I2C_Master_Transmit" writes 2 bytes of data in each loop - i2c

Question

I'm using HAL library for stm32f4 (V1.7.1) and trying to understand how HAL_I2C_Master_Transmit works. This function transmits a "master to slave" packet on SDA line.
In the stm32f4xx_hal_i2c.c code after sending slave address there is a loop (while(hi2c->XferSize > 0U)) that sends bytes which we want to be transmitted to slave. This loop works until all bytes are transmitted. But there is a question that "why the function wants to transmit TWO bytes in each loop?" There is an IF in loop (if((__HAL_I2C_GET_FLAG(hi2c, I2C_FLAG_BTF) == SET) && (hi2c->XferSize != 0U))) that checks if the previous byte is currently transferred then send the the next byte!
I don't know what is the reason of existing this IF when in the next loop other bytes can be transmitted?!
HAL_StatusTypeDef HAL_I2C_Master_Transmit(I2C_HandleTypeDef *hi2c, uint16_t
DevAddress, uint8_t *pData, uint16_t Size, uint32_t Timeout)
{
uint32_t tickstart = 0x00U;
/* Init tickstart for timeout management*/
tickstart = HAL_GetTick();
if(hi2c->State == HAL_I2C_STATE_READY)
{
/* Wait until BUSY flag is reset */
if(I2C_WaitOnFlagUntilTimeout(hi2c, I2C_FLAG_BUSY, SET,
I2C_TIMEOUT_BUSY_FLAG, tickstart) != HAL_OK)
{
return HAL_BUSY;
}
/* Process Locked */
__HAL_LOCK(hi2c);
/* Check if the I2C is already enabled */
if((hi2c->Instance->CR1 & I2C_CR1_PE) != I2C_CR1_PE)
{
/* Enable I2C peripheral */
__HAL_I2C_ENABLE(hi2c);
}
/* Disable Pos */
hi2c->Instance->CR1 &= ~I2C_CR1_POS;
hi2c->State = HAL_I2C_STATE_BUSY_TX;
hi2c->Mode = HAL_I2C_MODE_MASTER;
hi2c->ErrorCode = HAL_I2C_ERROR_NONE;
/* Prepare transfer parameters */
hi2c->pBuffPtr = pData;
hi2c->XferCount = Size;
hi2c->XferOptions = I2C_NO_OPTION_FRAME;
hi2c->XferSize = hi2c->XferCount;
/* Send Slave Address */
if(I2C_MasterRequestWrite(hi2c, DevAddress, Timeout, tickstart) != HAL_OK)
{
if(hi2c->ErrorCode == HAL_I2C_ERROR_AF)
{
/* Process Unlocked */
__HAL_UNLOCK(hi2c);
return HAL_ERROR;
}
else
{
/* Process Unlocked */
__HAL_UNLOCK(hi2c);
return HAL_TIMEOUT;
}
}
/* Clear ADDR flag */
__HAL_I2C_CLEAR_ADDRFLAG(hi2c);
while(hi2c->XferSize > 0U)
{
/* Wait until TXE flag is set */
if(I2C_WaitOnTXEFlagUntilTimeout(hi2c, Timeout, tickstart) != HAL_OK)
{
if(hi2c->ErrorCode == HAL_I2C_ERROR_AF)
{
/* Generate Stop */
hi2c->Instance->CR1 |= I2C_CR1_STOP;
return HAL_ERROR;
}
else
{
return HAL_TIMEOUT;
}
}
/* Write data to DR */
hi2c->Instance->DR = (*hi2c->pBuffPtr++);
hi2c->XferCount--;
hi2c->XferSize--;
if((__HAL_I2C_GET_FLAG(hi2c, I2C_FLAG_BTF) == SET) && (hi2c->XferSize != 0U))
{
/* Write data to DR */
hi2c->Instance->DR = (*hi2c->pBuffPtr++);
hi2c->XferCount--;
hi2c->XferSize--;
}
/* Wait until BTF flag is set */
if(I2C_WaitOnBTFFlagUntilTimeout(hi2c, Timeout, tickstart) != HAL_OK)
{
if(hi2c->ErrorCode == HAL_I2C_ERROR_AF)
{
/* Generate Stop */
hi2c->Instance->CR1 |= I2C_CR1_STOP;
return HAL_ERROR;
}
else
{
return HAL_TIMEOUT;
}
}
}
/* Generate Stop */
hi2c->Instance->CR1 |= I2C_CR1_STOP;
hi2c->State = HAL_I2C_STATE_READY;
hi2c->Mode = HAL_I2C_MODE_NONE;
/* Process Unlocked */
__HAL_UNLOCK(hi2c);
return HAL_OK;
}
else
{
return HAL_BUSY;
}
}