RS485 DE enable

[abhijith_raj]

Hi,
I am implementing RS-485 communication on the STM32G491. Based on available references, I’m currently using a custom GPIO pin for the DE (Driver Enable) signal, since the default USART1_DE pin is already allocated for another function. In the RS-485 initialization API, there are three parameters: DE polarity, assertion time, and de-assertion time. Do these parameters have any effect when a regular GPIO is used instead of the hardware-controlled DE pin?
I am using the below configuration for init.

huart1.Instance = USART1;
huart1.Init.BaudRate = 115200;
huart1.Init.WordLength = UART_WORDLENGTH_8B;
huart1.Init.StopBits = UART_STOPBITS_1;
huart1.Init.Parity = UART_PARITY_NONE;
huart1.Init.Mode = UART_MODE_TX_RX;
huart1.Init.HwFlowCtl = UART_HWCONTROL_NONE;
huart1.Init.OverSampling = UART_OVERSAMPLING_16;
huart1.Init.OneBitSampling = UART_ONE_BIT_SAMPLE_DISABLE;
huart1.Init.ClockPrescaler = UART_PRESCALER_DIV1;
huart1.AdvancedInit.AdvFeatureInit = UART_ADVFEATURE_MSBFIRST_INIT;
huart1.AdvancedInit.MSBFirst = UART_ADVFEATURE_MSBFIRST_ENABLE;
if (HAL_UART_Init(&huart1) != HAL_OK)
{
  Error_Handler();
}
if(HAL_RS485Ex_Init(&huart1, UART_DE_POLARITY_HIGH, 0x1E, 0x1E)!= HAL_OK)
{
  Error_Handler();
}

if (HAL_UARTEx_SetTxFifoThreshold(&huart1, UART_TXFIFO_THRESHOLD_1_8) != HAL_OK)
{
  Error_Handler();
}
if (HAL_UARTEx_SetRxFifoThreshold(&huart1, UART_RXFIFO_THRESHOLD_1_8) != HAL_OK)
{
  Error_Handler();
}
if (HAL_UARTEx_DisableFifoMode(&huart1) != HAL_OK)
{
  Error_Handler();
}

---

 Edited to apply source code formatting - please see How to insert source code for future reference

[MCORD.1]

Hello,

Here is an extract from the G4 series reference manual which should help you (available here) :

The driver enable feature is enabled by setting bit DEM in the USART_CR3 control register.
This enables the user to activate the external transceiver control, through the DE (Driver
Enable) signal. The assertion time is the time between the activation of the DE signal and
the beginning of the start bit. It is programmed using the DEAT [4:0] bitfields in the
USART_CR1 control register. The deassertion time is the time between the end of the last
stop bit, in a transmitted message, and the de-activation of the DE signal. It is programmed
using the DEDT [4:0] bitfields in the USART_CR1 control register. The polarity of the DE
signal can be configured using the DEP bit in the USART_CR3 control register.
In USART, the DEAT and DEDT are expressed in sample time units (1/8 or 1/16 bit time,
depending on the oversampling rate).

If you are planning on piloting the DE signal with a GPIO:

Polarity: defines the default state of the signal

Assertion time: time between the DE signal and the beginning of the UART signal

Deassertion time: time between end of the last stop bit signal and the beginning of the UART signal

You must be careful when configuring those time that both are expressed in sample time (in your case 1/16 of a bit time)

Hope that helps

Mathis 

[waclawek.jan · Accepted solution]

> Do these parameters have any effect when a regular GPIO is used instead of the hardware-controlled DE pin?

No, these govern the UART's built-in RS485 DE feature, which you don't intend to use.

JW